diff --git a/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowFlatMapConcatParallelismSpec.scala b/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowFlatMapConcatParallelismSpec.scala
index a36f9beab4..df7a7ddb14 100644
--- a/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowFlatMapConcatParallelismSpec.scala
+++ b/stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowFlatMapConcatParallelismSpec.scala
@@ -47,9 +47,12 @@ class FlowFlatMapConcatParallelismSpec extends StreamSpec("""
   val toSeq = Flow[Int].grouped(1000).toMat(Sink.head)(Keep.right)
 
   class BoomException extends RuntimeException("BOOM~~") with NoStackTrace
+
+  val checkValues = List(1, 2, 4, 8, 16, 32, 64, 128)
+
   "A flatMapConcat" must {
 
-    for (i <- 1 until 129) {
+    for (i <- checkValues) {
       s"work with value presented sources with parallelism: $i" in {
         Source(
           List(
@@ -98,7 +101,7 @@ class FlowFlatMapConcatParallelismSpec extends StreamSpec("""
       (sum, seq)
     }
 
-    for (i <- 1 until 129) {
+    for (i <- checkValues) {
       s"work with generated value presented sources with parallelism: $i " in {
         val (sum, sources @ _) = generateRandomValuePresentedSources(100000)
         Source(sources)
@@ -109,7 +112,7 @@ class FlowFlatMapConcatParallelismSpec extends StreamSpec("""
       }
     }
 
-    for (i <- 1 until 129) {
+    for (i <- checkValues) {
       s"work with generated value sequenced sources with parallelism: $i " in {
         val (sum, sources @ _) = generateSequencedValuePresentedSources(100000)
         Source(sources)