I've tried to keep it simple, but couldn't improve latency anymore. No matter if it's using socket.io or some faster network transport, there will be some latency. So I've used server's time as reference for clients to adjust the player's current time across devices.

However, there's still a delay added by the YouTube player when triggering the play action. It's an async action, and YouTube's embed player API _seekTo_ method seems to be rounding or not updating if the player is already playing in the nearby time. I'm not sure what else to do to achieve a perfect sync.