diff --git a/core/src/main/scala/ox/resilience/CircuitBreakerStateMachine.scala b/core/src/main/scala/ox/resilience/CircuitBreakerStateMachine.scala
index 21590389..2fd03286 100644
--- a/core/src/main/scala/ox/resilience/CircuitBreakerStateMachine.scala
+++ b/core/src/main/scala/ox/resilience/CircuitBreakerStateMachine.scala
@@ -5,10 +5,11 @@ import ox.*
 import java.util.concurrent.Semaphore
 import ox.channels.ActorRef
 import ox.resilience.CircuitBreakerStateMachine.nextState
+import scala.compiletime.ops.double
 
 private[resilience] case class CircuitBreakerStateMachine(
     config: CircuitBreakerStateMachineConfig,
-    results: CircuitBreakerResults
+    private val results: CircuitBreakerResults
 )(using val ox: Ox):
   @volatile private var _state: CircuitBreakerState = CircuitBreakerState.Closed
 
@@ -115,16 +116,22 @@ private[resilience] object CircuitBreakerResults:
     private var failedCalls = 0
     private var successCalls = 0
 
+    private def clearResults: Unit =
+      results.clear()
+      slowCalls = 0
+      failedCalls = 0
+      successCalls = 0
+
     def onStateChange(oldState: CircuitBreakerState, newState: CircuitBreakerState): Unit =
       import CircuitBreakerState.*
       // we have to match so we don't reset result when for example incrementing completed calls in halfopen state
       (oldState, newState) match
         case (Closed, Open(_) | HalfOpen(_, _, _)) =>
-          results.clear()
+          clearResults
         case (HalfOpen(_, _, _), Open(_) | Closed) =>
-          results.clear()
+          clearResults
         case (Open(_), Closed | HalfOpen(_, _, _)) =>
-          results.clear()
+          clearResults
         case (_, _) => ()
       end match
     end onStateChange
@@ -159,14 +166,31 @@ private[resilience] object CircuitBreakerResults:
 
   case class TimeWindowBased(windowDuration: FiniteDuration)(using ox: Ox) extends CircuitBreakerResults(using ox):
     // holds timestamp of recored operation and result
-    private val queue = collection.mutable.Queue[(Long, CircuitBreakerResult)]()
+    private val results = collection.mutable.ArrayDeque[(Long, CircuitBreakerResult)]()
+    private var slowCalls = 0
+    private var failedCalls = 0
+    private var successCalls = 0
+
+    private def clearResults(): Unit =
+      results.clear()
+      slowCalls = 0
+      failedCalls = 0
+      successCalls = 0
 
     def calculateMetrics(lastAcquisitionResult: Option[AcquireResult], timestamp: Long): Metrics =
       // filter all entries that happend outside sliding window
-      val results = queue.filterInPlace((time, _) => timestamp > time + windowDuration.toMillis)
-      val numOfOperations = results.length
-      val failuresRate = ((results.count(_ == CircuitBreakerResult.Failure) / numOfOperations.toFloat) * 100).toInt
-      val slowRate = ((results.count(_ == CircuitBreakerResult.Slow) / numOfOperations.toFloat) * 100).toInt
+      val res = results.filterInPlace { (time, result) =>
+        val isOlder = timestamp > time + windowDuration.toMillis
+        if isOlder then
+          result match
+            case CircuitBreakerResult.Success => successCalls -= 1
+            case CircuitBreakerResult.Failure => failedCalls -= 1
+            case CircuitBreakerResult.Slow    => slowCalls -= 1
+        isOlder
+      }
+      val numOfOperations = res.length
+      val failuresRate = ((failedCalls / numOfOperations.toFloat) * 100).toInt
+      val slowRate = ((slowCalls / numOfOperations.toFloat) * 100).toInt
       Metrics(
         failuresRate,
         slowRate,
@@ -177,18 +201,22 @@ private[resilience] object CircuitBreakerResults:
     end calculateMetrics
 
     def updateResults(result: CircuitBreakerResult): Unit =
-      queue.addOne((System.currentTimeMillis(), result))
+      result match
+        case CircuitBreakerResult.Success => successCalls += 1
+        case CircuitBreakerResult.Failure => failedCalls += 1
+        case CircuitBreakerResult.Slow    => slowCalls += 1
+      results.addOne((System.currentTimeMillis(), result))
 
     def onStateChange(oldState: CircuitBreakerState, newState: CircuitBreakerState): Unit =
       import CircuitBreakerState.*
       // we have to match so we don't reset result when for example incrementing completed calls in halfopen state
       (oldState, newState) match
         case (Closed, Open(_) | HalfOpen(_, _, _)) =>
-          queue.clear()
+          clearResults()
         case (HalfOpen(_, _, _), Open(_) | Closed) =>
-          queue.clear()
+          clearResults()
         case (Open(_), Closed | HalfOpen(_, _, _)) =>
-          queue.clear()
+          clearResults()
         case (_, _) => ()
       end match
     end onStateChange
diff --git a/core/src/test/scala/ox/resilience/CircuitBreakerTest.scala b/core/src/test/scala/ox/resilience/CircuitBreakerTest.scala
index 854705af..ee12731f 100644
--- a/core/src/test/scala/ox/resilience/CircuitBreakerTest.scala
+++ b/core/src/test/scala/ox/resilience/CircuitBreakerTest.scala
@@ -133,8 +133,8 @@ class CircuitBreakerTest extends AnyFlatSpec with Matchers with OptionValues wit
         minimumNumberOfCalls = numberOfOperations,
         slidingWindow = SlidingWindow.CountBased(numberOfOperations),
         numberOfCallsInHalfOpenState = 1,
-        waitDurationOpenState = 10.millis,
-        halfOpenTimeoutDuration = 1.second
+        waitDurationOpenState = 1.seconds,
+        halfOpenTimeoutDuration = 2.seconds
       )
     )
     def f(): Either[String, String] =
@@ -142,9 +142,9 @@ class CircuitBreakerTest extends AnyFlatSpec with Matchers with OptionValues wit
 
     // when
     val result1 = circuitBreaker.runOrDropEither(f()) // trigger swithc to open
-    sleep(100.millis) // wait for state to register, and for switch to half open
+    sleep(1500.millis) // wait for state to register, and for switch to half open
     val state = circuitBreaker.stateMachine.state
-    sleep(1500.millis) // wait longer than half open timeout
+    sleep(2500.millis) // wait longer than half open timeout
     val stateAfterWait = circuitBreaker.stateMachine.state
 
     // then