diff --git a/internal/flowcontrol/connection_flow_controller.go b/internal/flowcontrol/connection_flow_controller.go
index 934d646de..351458e40 100644
--- a/internal/flowcontrol/connection_flow_controller.go
+++ b/internal/flowcontrol/connection_flow_controller.go
@@ -54,13 +54,12 @@ func (c *connectionFlowController) IncrementHighestReceived(increment protocol.B
 
 func (c *connectionFlowController) GetWindowUpdate() protocol.ByteCount {
 	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	oldWindowIncrement := c.receiveWindowIncrement
 	offset := c.baseFlowController.getWindowUpdate()
 	if oldWindowIncrement < c.receiveWindowIncrement {
 		utils.Debugf("Increasing receive flow control window for the connection to %d kB", c.receiveWindowIncrement/(1<<10))
 	}
+	c.mutex.Unlock()
 	return offset
 }
 
@@ -68,10 +67,9 @@ func (c *connectionFlowController) GetWindowUpdate() protocol.ByteCount {
 // it should make sure that the connection-level window is increased when a stream-level window grows
 func (c *connectionFlowController) EnsureMinimumWindowIncrement(inc protocol.ByteCount) {
 	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	if inc > c.receiveWindowIncrement {
 		c.receiveWindowIncrement = utils.MinByteCount(inc, c.maxReceiveWindowIncrement)
 		c.lastWindowUpdateTime = time.Time{} // disables autotuning for the next window update
 	}
+	c.mutex.Unlock()
 }
diff --git a/internal/flowcontrol/stream_flow_controller.go b/internal/flowcontrol/stream_flow_controller.go
index dadba72e5..2bbf3dcae 100644
--- a/internal/flowcontrol/stream_flow_controller.go
+++ b/internal/flowcontrol/stream_flow_controller.go
@@ -113,11 +113,11 @@ func (c *streamFlowController) SendWindowSize() protocol.ByteCount {
 }
 
 func (c *streamFlowController) GetWindowUpdate() protocol.ByteCount {
+	// don't use defer for unlocking the mutex here, GetWindowUpdate() is called frequently and defer shows up in the profiler
 	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	// if we already received the final offset for this stream, the peer won't need any additional flow control credit
 	if c.receivedFinalOffset {
+		c.mutex.Unlock()
 		return 0
 	}
 
@@ -129,5 +129,6 @@ func (c *streamFlowController) GetWindowUpdate() protocol.ByteCount {
 			c.connection.EnsureMinimumWindowIncrement(protocol.ByteCount(float64(c.receiveWindowIncrement) * protocol.ConnectionFlowControlMultiplier))
 		}
 	}
+	c.mutex.Unlock()
 	return offset
 }