diff --git a/akka-actor-tests/src/test/scala/akka/routing/MetricsBasedResizerSpec.scala b/akka-actor-tests/src/test/scala/akka/routing/MetricsBasedResizerSpec.scala
index eb126235b7..c2524233e7 100644
--- a/akka-actor-tests/src/test/scala/akka/routing/MetricsBasedResizerSpec.scala
+++ b/akka-actor-tests/src/test/scala/akka/routing/MetricsBasedResizerSpec.scala
@@ -241,7 +241,7 @@ class MetricsBasedResizerSpec extends AkkaSpec(ResizerSpec.config) with DefaultT
       msgs1.foreach(_.second.open()) //process two messages
 
       // make sure some time passes inbetween
-      Thread.sleep(100)
+      Thread.sleep(300)
 
       // wait for routees to update their mail boxes
       msgs2.foreach(l ⇒ Await.ready(l.first, timeout.duration))
@@ -273,7 +273,7 @@ class MetricsBasedResizerSpec extends AkkaSpec(ResizerSpec.config) with DefaultT
       msgs1.foreach(_.second.open()) //process two messages
 
       // make sure some time passes inbetween
-      Thread.sleep(100)
+      Thread.sleep(300)
 
       // wait for routees to update their mail boxes
       msgs2.foreach(l ⇒ Await.ready(l.first, timeout.duration))
diff --git a/akka-actor/src/main/scala/akka/routing/OptimalSizeExploringResizer.scala b/akka-actor/src/main/scala/akka/routing/OptimalSizeExploringResizer.scala
index 51848aa702..ac09e227f4 100644
--- a/akka-actor/src/main/scala/akka/routing/OptimalSizeExploringResizer.scala
+++ b/akka-actor/src/main/scala/akka/routing/OptimalSizeExploringResizer.scala
@@ -7,14 +7,13 @@ import java.time.LocalDateTime
 
 import scala.collection.immutable
 import java.util.concurrent.ThreadLocalRandom
+
 import scala.concurrent.duration._
-
 import com.typesafe.config.Config
-
 import akka.actor._
 import akka.util.JavaDurationConverters._
-
 import OptimalSizeExploringResizer._
+import akka.annotation.InternalApi
 
 trait OptimalSizeExploringResizer extends Resizer {
   /**
@@ -126,17 +125,26 @@ case class DefaultOptimalSizeExploringResizer(
   explorationProbability:                         Double   = 0.4,
   weightOfLatestMetric:                           Double   = 0.5) extends OptimalSizeExploringResizer {
   /**
+   * INTERNAL API
+   *
    * Leave package accessible for testing purpose
    */
+  @InternalApi
   private[routing] var performanceLog: PerformanceLog = Map.empty
   /**
+   * INTERNAL API
+   *
    * Leave package accessible for testing purpose
    */
+  @InternalApi
   private[routing] var record: ResizeRecord = ResizeRecord()
 
   /**
+   * INTERNAL API
+   *
    * Leave package accessible for testing purpose
    */
+  @InternalApi
   private[routing] var stopExploring = false
 
   private def random = ThreadLocalRandom.current()