diff --git a/transport/internet/kcp/connection.go b/transport/internet/kcp/connection.go
index 4cf82736..4b7d92dc 100644
--- a/transport/internet/kcp/connection.go
+++ b/transport/internet/kcp/connection.go
@@ -343,6 +343,9 @@ func (c *Connection) waitForDataOutput() error {
 
 // Write implements io.Writer.
 func (c *Connection) Write(b []byte) (int, error) {
+	// This involves multiple copies of the buffer. But we don't expect this method to be used often.
+	// Only wrapped connections such as TLS and WebSocket will call into this.
+	// TODO: improve effeciency.
 	var mb buf.MultiBuffer
 	common.Must2(mb.Write(b))
 	if err := c.WriteMultiBuffer(mb); err != nil {