Greasy Fork

It works like a charm, the stream plays smoother after using the script.

But Chat Box is still the most compute costuming event, and brings notable lag in popular live streams. If you want a video-like smoothness when watching streams, you can try pop out and minimize the chat box, while using something like the "Flow YouTube Chat" script to keep up with viewers.

I further found some coding issues in YouTube's scripts. I created a script to aggressively fix these issues. You can try to install and see where it works for you or not.

YouTube Super Fast Chat: https://greasyfork.org/en/scripts/469878-youtube-super-fast-chat

Unlike this script, YouTube Super Fast Chat changes the way it renders the chat messages, for example, disable the animation for new messages (i.e. smooth scrolling is removed)

YouTube Super Fast Chat is further updated and notable lags shall disappear.

Please check that script and give good review there if possible.