diff --git a/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java b/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java
index 13af0c4b356f..db8127f473ce 100644
--- a/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java
+++ b/sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/GroupByKeyTest.java
@@ -237,10 +237,11 @@ public void testAfterProcessingTimeContinuationTriggerEarly() throws Exception {
      * many runners do not support it.
      *
      * <p>Since a delay in processing time is fundamental to this test, it unfortunately must have a
-     * real-time sleep. Currently the sleep is sub-second so it is acceptable delay for a unit test
-     * suite.
+     * real-time sleep. Currently the sleep is a couple of seconds so it is an acceptable delay for
+     * a unit test suite.
      */
     @Test
+    @BeamParallelJunit4Runner.SerialTest
     @Category({
       ValidatesRunner.class,
       UsesTimersInParDo.class,
@@ -249,7 +250,7 @@ public void testAfterProcessingTimeContinuationTriggerEarly() throws Exception {
     })
     public void testAfterProcessingTimeContinuationTriggerUsingState() throws Exception {
       final long triggerMillis = 1;
-      final long waitMillis = 500;
+      final long waitMillis = 2000;
 
       PCollection<Integer> triggeredSums =
           p.apply(