diff --git a/flowcontrol/flow_controller.go b/flowcontrol/flow_controller.go
index 343eaa58..d242f878 100644
--- a/flowcontrol/flow_controller.go
+++ b/flowcontrol/flow_controller.go
@@ -1,8 +1,6 @@
 package flowcontrol
 
 import (
-	"sync"
-
 	"github.com/lucas-clemente/quic-go/handshake"
 	"github.com/lucas-clemente/quic-go/protocol"
 )
@@ -19,8 +17,6 @@ type flowController struct {
 	highestReceived                   protocol.ByteCount
 	receiveFlowControlWindow          protocol.ByteCount
 	receiveFlowControlWindowIncrement protocol.ByteCount
-
-	mutex sync.RWMutex
 }
 
 // newFlowController gets a new flow controller
@@ -52,18 +48,12 @@ func (c *flowController) getSendFlowControlWindow() protocol.ByteCount {
 }
 
 func (c *flowController) AddBytesSent(n protocol.ByteCount) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	c.bytesSent += n
 }
 
 // UpdateSendWindow should be called after receiving a WindowUpdateFrame
 // it returns true if the window was actually updated
 func (c *flowController) UpdateSendWindow(newOffset protocol.ByteCount) bool {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	if newOffset > c.sendFlowControlWindow {
 		c.sendFlowControlWindow = newOffset
 		return true
@@ -73,9 +63,6 @@ func (c *flowController) UpdateSendWindow(newOffset protocol.ByteCount) bool {
 
 // TODO: remove once the Stream doesn't use it anymore
 func (c *flowController) SendWindowSize() protocol.ByteCount {
-	c.mutex.RLock()
-	defer c.mutex.RUnlock()
-
 	sendFlowControlWindow := c.getSendFlowControlWindow()
 
 	if c.bytesSent > sendFlowControlWindow { // should never happen, but make sure we don't do an underflow here
@@ -85,18 +72,12 @@ func (c *flowController) SendWindowSize() protocol.ByteCount {
 }
 
 func (c *flowController) SendWindowOffset() protocol.ByteCount {
-	c.mutex.RLock()
-	defer c.mutex.RUnlock()
-
 	return c.getSendFlowControlWindow()
 }
 
 // UpdateHighestReceived updates the highestReceived value, if the byteOffset is higher
 // Should **only** be used for the stream-level FlowController
 func (c *flowController) UpdateHighestReceived(byteOffset protocol.ByteCount) protocol.ByteCount {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	if byteOffset > c.highestReceived {
 		increment := byteOffset - c.highestReceived
 		c.highestReceived = byteOffset
@@ -108,25 +89,16 @@ func (c *flowController) UpdateHighestReceived(byteOffset protocol.ByteCount) pr
 // IncrementHighestReceived adds an increment to the highestReceived value
 // Should **only** be used for the connection-level FlowController
 func (c *flowController) IncrementHighestReceived(increment protocol.ByteCount) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	c.highestReceived += increment
 }
 
 func (c *flowController) AddBytesRead(n protocol.ByteCount) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	c.bytesRead += n
 }
 
 // MaybeTriggerWindowUpdate determines if it is necessary to send a WindowUpdate
 // if so, it returns true and the offset of the window
 func (c *flowController) MaybeTriggerWindowUpdate() (bool, protocol.ByteCount) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	diff := c.receiveFlowControlWindow - c.bytesRead
 	// Chromium implements the same threshold
 	if diff < (c.receiveFlowControlWindowIncrement / 2) {
@@ -137,9 +109,6 @@ func (c *flowController) MaybeTriggerWindowUpdate() (bool, protocol.ByteCount) {
 }
 
 func (c *flowController) CheckFlowControlViolation() bool {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-
 	if c.highestReceived > c.receiveFlowControlWindow {
 		return true
 	}