perf(group): reduce websocket fanout serialization

server/datamodel.go

@@ -881,6 +881,8 @@ type ServerComMessage struct {
 	PreserveTopicName bool `json:"-"`
 	// User id affected by this message.
 	uid types.Uid
+	// Timestamp when this message entered a master-to-proxy multiplex queue.
+	clusterQueuedAt time.Time
 }
 
 // Deep-shallow copy of ServerComMessage. Deep copy of service fields,
@@ -899,6 +901,7 @@ func (src *ServerComMessage) copy() *ServerComMessage {
 		SkipSid:           src.SkipSid,
 		PreserveTopicName: src.PreserveTopicName,
 		uid:               src.uid,
+		clusterQueuedAt:   src.clusterQueuedAt,
 	}
 
 	dst.Ctrl = src.Ctrl.copy()