diff --git a/build.gradle b/build.gradle
index ce00bbd208b..61f25361f56 100644
--- a/build.gradle
+++ b/build.gradle
@@ -47,7 +47,14 @@ ext {
     // needing --rerun-tasks. Useful for repeatedly running the same test command while
     // chasing flaky tests across runs.
     doNotCacheTests = System.getenv('DO_NOT_CACHE_TESTS')?.toBoolean()
-    configuredTestParallel = findProperty('maxTestParallel') as Integer ?: (isCiBuild ? 4 : Runtime.runtime.availableProcessors() * 3 / 4 as int ?: 1)
+    // availableProcessors() reports LOGICAL processors: SMT threads on x64, and on Apple silicon
+    // every efficiency core as well as every performance core. Taking 3/4 of that therefore
+    // asked for more concurrent Grails/Spring test JVMs than the machine has real capacity for
+    // (each fork also holds a 1G heap, and the mongodb, redis and geb suites start a Docker
+    // container per fork). Half is the conservative fraction. CI keeps its existing budget, so
+    // this does not reduce CI fork counts - though CI forks do see fewer processors, below.
+    configuredTestParallel = findProperty('maxTestParallel') as Integer ?:
+            (isCiBuild ? 4 : Math.max(1, (Runtime.runtime.availableProcessors() / 2) as int))
     excludeUnusedTransDeps = findProperty('excludeUnusedTransDeps')
 
     testProjectsStartWith = [
@@ -60,8 +67,53 @@ ext {
     profileProjects = [ /* Will be populated by subprojects loop below */]
 }
 
+/**
+ * Tells a forked test JVM how many processors it may size its GC and JIT thread pools from.
+ *
+ * Every forked JVM otherwise sizes those pools for the WHOLE machine, because no fork knows the
+ * others exist. On a 20-processor host that is 15 ParallelGCThreads + 4 ConcGCThreads + 12
+ * CICompilerCount = 31 threads per fork before a single test runs, so N concurrent forks become
+ * N*31 threads competing for the same cores. Gradle cannot see this: it charges each fork one
+ * worker lease, as though a fork were a single thread.
+ *
+ * The share is availableProcessors / maxWorkerCount, i.e. the BUILD-WIDE worker limit, because
+ * that - not any one task's maxParallelForks - is what bounds how many forks can run at once.
+ * With org.gradle.parallel=true, Test tasks from several projects execute concurrently, so a
+ * per-task share would still oversubscribe the machine.
+ *
+ * Supplied as an argument provider rather than through jvmArgs because several modules assign
+ * jvmArgs wholesale in their own build scripts, which would discard anything added here first.
+ */
+final class ActiveProcessorCountArgumentProvider implements CommandLineArgumentProvider {
+
+    @Input
+    final int processorCount
+
+    ActiveProcessorCountArgumentProvider(int availableProcessorCount, int maxWorkerCount) {
+        // Floor of 2: at ActiveProcessorCount=1 HotSpot's ergonomics select Serial GC, which
+        // costs more on a 1G test heap than the contention it saves. Two keeps G1 with a
+        // bounded worker count.
+        //
+        // That floor also means -PmaxTestParallel=1 still advertises 2 processors to the
+        // single fork whenever maxWorkerCount is larger than availableProcessors/2 (the
+        // usual local case: org.gradle.parallel=true with no org.gradle.workers.max).
+        // It is not a "give the one fork the whole machine" switch.
+        this.processorCount = Math.max(2, (availableProcessorCount / Math.max(1, maxWorkerCount)) as int)
+    }
+
+    @Override
+    Iterable<String> asArguments() {
+        ["-XX:ActiveProcessorCount=$processorCount".toString()]
+    }
+}
+
 subprojects {
 
+    tasks.withType(Test).configureEach { testTask ->
+        testTask.jvmArgumentProviders.add(new ActiveProcessorCountArgumentProvider(
+                Runtime.runtime.availableProcessors(), gradle.startParameter.maxWorkerCount))
+    }
+
     for (String testPrefix : testProjectsStartWith) {
         if (name.startsWith(testPrefix)) {
             rootProject.ext['testProjects'] << name
diff --git a/gradle/functional-test-config.gradle b/gradle/functional-test-config.gradle
index c92ccc76072..7b11bbdfa28 100644
--- a/gradle/functional-test-config.gradle
+++ b/gradle/functional-test-config.gradle
@@ -170,7 +170,7 @@ tasks.withType(Test).configureEach { Test task ->
         showStackTraces = true
     }
     if (findProperty('testForked')) {
-        task.maxParallelForks = configuredTestParallel
+        task.maxParallelForks = rootProject.ext.configuredTestParallel
         task.forkEvery = (findProperty('testForkEvery') ?: 0)
     }
     if (findProperty('testJvmArgs')) {
diff --git a/gradle/mongodb-forked-test-config.gradle b/gradle/mongodb-forked-test-config.gradle
index 77f27033227..947665619cb 100644
--- a/gradle/mongodb-forked-test-config.gradle
+++ b/gradle/mongodb-forked-test-config.gradle
@@ -50,7 +50,7 @@ tasks.withType(Test).configureEach {
     }
 
     useJUnitPlatform()
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     jvmArgs = ['-Xmx768M']
     addTestListener([
         afterSuite: { desc, result ->
@@ -67,4 +67,4 @@ tasks.withType(Test).configureEach {
     }
     // Used in the TCK test to selectively enable/disable tests
     systemProperty('mongodb.gorm.suite', 'true')
-}
\ No newline at end of file
+}
diff --git a/gradle/test-config.gradle b/gradle/test-config.gradle
index e4ad66b9c5a..d6ea685c670 100644
--- a/gradle/test-config.gradle
+++ b/gradle/test-config.gradle
@@ -83,7 +83,7 @@ tasks.withType(Test).configureEach {
         showStackTraces = true
     }
     excludes = ['**/*TestCase.class', '**/*$*.class']
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     maxHeapSize = isCiBuild ? '768m' : '1024m'
     forkEvery = hasProperty('forkEveryUnitTest') ? getProperty('forkEveryUnitTest') as long : (isCiBuild ? 50 : 100)
     if (System.getProperty('debug.tests')) {
diff --git a/grails-core/src/test/groovy/grails/util/ActiveProcessorCountForkTests.java b/grails-core/src/test/groovy/grails/util/ActiveProcessorCountForkTests.java
new file mode 100644
index 00000000000..fd1cf7d537f
--- /dev/null
+++ b/grails-core/src/test/groovy/grails/util/ActiveProcessorCountForkTests.java
@@ -0,0 +1,58 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one or more
+ *  contributor license agreements.  See the NOTICE file distributed with
+ *  this work for additional information regarding copyright ownership.
+ *  The ASF licenses this file to You under the Apache License, Version 2.0
+ *  (the "License"); you may not use this file except in compliance with
+ *  the License.  You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package grails.util;
+
+import java.lang.management.ManagementFactory;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Asserts that {@code -XX:ActiveProcessorCount} from the root build's
+ * {@code jvmArgumentProviders} actually reaches this forked test JVM.
+ * Wholesale {@code jvmArgs = ...} assignments in module scripts would drop a
+ * flag added via {@code jvmArgs}, which is why the build uses a provider.
+ */
+class ActiveProcessorCountForkTests {
+
+    private static final String FLAG_PREFIX = "-XX:ActiveProcessorCount=";
+
+    @Test
+    void forkedTestJvmReceivesActiveProcessorCount() {
+        String flag = findActiveProcessorCountFlag();
+        assertNotNull(flag,
+                "forked test JVM must receive -XX:ActiveProcessorCount from jvmArgumentProviders");
+        int advertised = Integer.parseInt(flag.substring(FLAG_PREFIX.length()));
+        assertTrue(advertised >= 2, "floor is 2 so HotSpot keeps G1: " + advertised);
+        assertEquals(advertised, Runtime.getRuntime().availableProcessors(),
+                "HotSpot must honour the advertised processor count");
+    }
+
+    private static String findActiveProcessorCountFlag() {
+        List<String> args = ManagementFactory.getRuntimeMXBean().getInputArguments();
+        for (String arg : args) {
+            if (arg.startsWith(FLAG_PREFIX)) {
+                return arg;
+            }
+        }
+        return null;
+    }
+}
diff --git a/grails-data-neo4j/grails-datastore-gorm-neo4j/build.gradle b/grails-data-neo4j/grails-datastore-gorm-neo4j/build.gradle
index 8267779b0a8..852ef75dc07 100644
--- a/grails-data-neo4j/grails-datastore-gorm-neo4j/build.gradle
+++ b/grails-data-neo4j/grails-datastore-gorm-neo4j/build.gradle
@@ -47,7 +47,7 @@ dependencies {
 
 test {
     useJUnitPlatform()
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     forkEvery = 10
 
     jvmArgs = ['-Xmx1028M']
diff --git a/grails-forge/build.gradle b/grails-forge/build.gradle
index d262f97fab9..a149ee6e110 100644
--- a/grails-forge/build.gradle
+++ b/grails-forge/build.gradle
@@ -42,10 +42,32 @@ ext {
     // needing --rerun-tasks. Useful for repeatedly running the same test command while
     // chasing flaky tests across runs.
     doNotCacheTests = System.getenv('DO_NOT_CACHE_TESTS')?.toBoolean()
+    // Mirror grails-gradle: honour -PmaxTestParallel, and cap CI forks so a 4-vCPU
+    // runner does not inherit the local "half the logical processors" formula.
+    configuredTestParallel = findProperty('maxTestParallel') as Integer ?:
+            (isCiBuild ? 3 : Math.max(1, (Runtime.runtime.availableProcessors() / 2) as int))
+}
+
+/** Caps each test fork's view of the machine. See the root build.gradle for the rationale. */
+final class ActiveProcessorCountArgumentProvider implements CommandLineArgumentProvider {
+
+    @Input
+    final int processorCount
+
+    ActiveProcessorCountArgumentProvider(int availableProcessorCount, int maxWorkerCount) {
+        // Floor of 2 keeps G1. -PmaxTestParallel=1 still sees 2 processors when
+        // maxWorkerCount is the usual unrestricted local worker pool.
+        this.processorCount = Math.max(2, (availableProcessorCount / Math.max(1, maxWorkerCount)) as int)
+    }
+
+    @Override
+    Iterable<String> asArguments() {
+        ["-XX:ActiveProcessorCount=$processorCount".toString()]
+    }
 }
 
 allprojects {
-    tasks.withType(Test).configureEach { Task testTask ->
+    tasks.withType(Test).configureEach { testTask ->
         testTask.dependsOn(
                 gradle.includedBuild('grails-core').task(':publishAllPublicationsToTestCaseMavenRepoRepository'),
                 gradle.includedBuild('grails-gradle').task(':publishAllPublicationsToTestCaseMavenRepoRepository')
@@ -55,6 +77,8 @@ allprojects {
         // without --rerun-tasks (and without recompiling everything else).
         testTask.outputs.cacheIf { !doNotCacheTests }
         testTask.outputs.upToDateWhen { !doNotCacheTests }
+        testTask.jvmArgumentProviders.add(new ActiveProcessorCountArgumentProvider(
+                Runtime.runtime.availableProcessors(), gradle.startParameter.maxWorkerCount))
     }
 }
 
@@ -72,4 +96,4 @@ apply {
     // we must apply the publish configuration first or the docs config will not work
     from layout.projectDirectory.file('gradle/publish-root-config.gradle')
     from layout.projectDirectory.file('gradle/gradle-wrapper-root-config.gradle')
-}
\ No newline at end of file
+}
diff --git a/grails-forge/gradle/test-config.gradle b/grails-forge/gradle/test-config.gradle
index 9d4d27ddf9c..5f2df2b80b2 100644
--- a/grails-forge/gradle/test-config.gradle
+++ b/grails-forge/gradle/test-config.gradle
@@ -49,7 +49,7 @@ tasks.withType(Test).configureEach {
     jvmArgs('-Duser.country=US', '-Duser.language=en',
             '--add-opens', 'java.base/java.lang=ALL-UNNAMED')
     forkEvery = 100
-    maxParallelForks = Runtime.runtime.availableProcessors().intdiv(2) ?: 1
+    maxParallelForks = rootProject.ext.configuredTestParallel
     maxHeapSize = '2G'
 
     useJUnitPlatform()
diff --git a/grails-forge/grails-forge-core/src/test/groovy/org/grails/forge/ActiveProcessorCountForkSpec.groovy b/grails-forge/grails-forge-core/src/test/groovy/org/grails/forge/ActiveProcessorCountForkSpec.groovy
new file mode 100644
index 00000000000..f1f9ea3bfa2
--- /dev/null
+++ b/grails-forge/grails-forge-core/src/test/groovy/org/grails/forge/ActiveProcessorCountForkSpec.groovy
@@ -0,0 +1,39 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one
+ *  or more contributor license agreements.  See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership.  The ASF licenses this file
+ *  to you under the Apache License, Version 2.0 (the
+ *  "License"); you may not use this file except in compliance
+ *  with the License.  You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing,
+ *  software distributed under the License is distributed on an
+ *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ *  KIND, either express or implied.  See the License for the
+ *  specific language governing permissions and limitations
+ *  under the License.
+ */
+package org.grails.forge
+
+import java.lang.management.ManagementFactory
+
+import spock.lang.Specification
+
+class ActiveProcessorCountForkSpec extends Specification {
+
+    private static final String FLAG_PREFIX = '-XX:ActiveProcessorCount='
+
+    void 'forked test JVM receives ActiveProcessorCount from jvmArgumentProviders'() {
+        given:
+        String flag = ManagementFactory.runtimeMXBean.inputArguments.find { it.startsWith(FLAG_PREFIX) }
+
+        expect:
+        flag != null
+        int advertised = Integer.parseInt(flag.substring(FLAG_PREFIX.length()))
+        advertised >= 2
+        Runtime.runtime.availableProcessors() == advertised
+    }
+}
diff --git a/grails-gradle/build.gradle b/grails-gradle/build.gradle
index 382c9771024..bc42c184e2c 100644
--- a/grails-gradle/build.gradle
+++ b/grails-gradle/build.gradle
@@ -47,7 +47,35 @@ ext {
     // needing --rerun-tasks. Useful for repeatedly running the same test command while
     // chasing flaky tests across runs.
     doNotCacheTests = System.getenv('DO_NOT_CACHE_TESTS')?.toBoolean()
-    configuredTestParallel = findProperty('maxTestParallel') as Integer ?: (isCiBuild ? 3 : Runtime.runtime.availableProcessors() * 3/4 as int ?: 1)
+    // Half the LOGICAL processors, not 3/4. This separate Gradle build mirrors the root
+    // build.gradle - see it for the full rationale. CI keeps its existing budget.
+    configuredTestParallel = findProperty('maxTestParallel') as Integer ?:
+            (isCiBuild ? 3 : Math.max(1, (Runtime.runtime.availableProcessors() / 2) as int))
+}
+
+/** Caps each test fork's view of the machine. See the root build.gradle for the rationale. */
+final class ActiveProcessorCountArgumentProvider implements CommandLineArgumentProvider {
+
+    @Input
+    final int processorCount
+
+    ActiveProcessorCountArgumentProvider(int availableProcessorCount, int maxWorkerCount) {
+        // Floor of 2 keeps G1. -PmaxTestParallel=1 still sees 2 processors when
+        // maxWorkerCount is the usual unrestricted local worker pool.
+        this.processorCount = Math.max(2, (availableProcessorCount / Math.max(1, maxWorkerCount)) as int)
+    }
+
+    @Override
+    Iterable<String> asArguments() {
+        ["-XX:ActiveProcessorCount=$processorCount".toString()]
+    }
+}
+
+subprojects {
+    tasks.withType(Test).configureEach { testTask ->
+        testTask.jvmArgumentProviders.add(new ActiveProcessorCountArgumentProvider(
+                Runtime.runtime.availableProcessors(), gradle.startParameter.maxWorkerCount))
+    }
 }
 
 apply {
diff --git a/grails-gradle/gradle/test-config.gradle b/grails-gradle/gradle/test-config.gradle
index 8011f65a069..c6662764c80 100644
--- a/grails-gradle/gradle/test-config.gradle
+++ b/grails-gradle/gradle/test-config.gradle
@@ -59,7 +59,7 @@ tasks.withType(Test).configureEach {
         showStackTraces = true
     }
     excludes = ['**/*TestCase.class', '**/*$*.class']
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     maxHeapSize = isCiBuild ? '768m' : '1024m'
     if (System.getProperty('debug.tests')) {
         jvmArgs += debugArguments
diff --git a/grails-gradle/plugins/src/test/groovy/org/grails/gradle/plugin/core/ActiveProcessorCountForkSpec.groovy b/grails-gradle/plugins/src/test/groovy/org/grails/gradle/plugin/core/ActiveProcessorCountForkSpec.groovy
new file mode 100644
index 00000000000..051603402a7
--- /dev/null
+++ b/grails-gradle/plugins/src/test/groovy/org/grails/gradle/plugin/core/ActiveProcessorCountForkSpec.groovy
@@ -0,0 +1,38 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one
+ *  or more contributor license agreements.  See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership.  The ASF licenses this file
+ *  to you under the Apache License, Version 2.0 (the
+ *  "License"); you may not use this file except in compliance
+ *  with the License.  You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.grails.gradle.plugin.core
+
+import java.lang.management.ManagementFactory
+
+import spock.lang.Specification
+
+class ActiveProcessorCountForkSpec extends Specification {
+
+    private static final String FLAG_PREFIX = '-XX:ActiveProcessorCount='
+
+    void 'forked test JVM receives ActiveProcessorCount from jvmArgumentProviders'() {
+        given:
+        String flag = ManagementFactory.runtimeMXBean.inputArguments.find { it.startsWith(FLAG_PREFIX) }
+
+        expect:
+        flag != null
+        int advertised = Integer.parseInt(flag.substring(FLAG_PREFIX.length()))
+        advertised >= 2
+        Runtime.runtime.availableProcessors() == advertised
+    }
+}
diff --git a/grails-test-suite-persistence/build.gradle b/grails-test-suite-persistence/build.gradle
index 73173a79c56..476fd2609b1 100644
--- a/grails-test-suite-persistence/build.gradle
+++ b/grails-test-suite-persistence/build.gradle
@@ -88,7 +88,7 @@ dependencies {
 }
 
 test {
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     forkEvery = isCiBuild ? 25 : 100
     if(!isCiBuild) {
         maxHeapSize = '2048m'
diff --git a/grails-test-suite-uber/build.gradle b/grails-test-suite-uber/build.gradle
index 4fc49f517d5..74369b5e5bd 100644
--- a/grails-test-suite-uber/build.gradle
+++ b/grails-test-suite-uber/build.gradle
@@ -122,11 +122,6 @@ isolatedTestPatterns.keySet().each { taskName ->
     }
 }
 
-tasks.named('isolatedTestsTwo', Test) {
-    maxParallelForks = 1
-    forkEvery = 100
-}
-
 tasks.withType(Test).configureEach {
     // Honor DO_NOT_CACHE_TESTS=1 so developers can repeatedly invoke the same test command
     // without --rerun-tasks (and without recompiling everything else).
@@ -135,12 +130,20 @@ tasks.withType(Test).configureEach {
 
     onlyIf { !testSkippingProperties.any {project.hasProperty(it) } }
     useJUnitPlatform()
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     forkEvery = isCiBuild ? 50 : 100
     maxHeapSize = isCiBuild ? '768m' : '1024m'
     jvmArgs('--add-opens=java.base/java.lang=ALL-UNNAMED', '--add-opens=java.base/java.util=ALL-UNNAMED')
 }
 
+// Must be configured AFTER the tasks.withType(Test) block above. Both are deferred
+// configuration actions and Gradle runs them in registration order, so a tasks.named
+// block registered first would be silently overwritten by the later configureEach.
+tasks.named('isolatedTestsTwo', Test) {
+    maxParallelForks = 1
+    forkEvery = 100
+}
+
 tasks.named('test', Test) {
     // Exclude the isolated tests from the main test task
     filter.excludePatterns = isolatedTestPatterns.values().flatten()
diff --git a/grails-test-suite-web/build.gradle b/grails-test-suite-web/build.gradle
index 140465335c7..8281c284b53 100644
--- a/grails-test-suite-web/build.gradle
+++ b/grails-test-suite-web/build.gradle
@@ -53,7 +53,7 @@ dependencies {
 }
 
 def defaultTestConfig = {
-    maxParallelForks = configuredTestParallel
+    maxParallelForks = rootProject.ext.configuredTestParallel
     forkEvery = isCiBuild ? 50 : 100
     excludes = ['**/*TestCase.class', '**/*$*.class']
 }