If it's so performance critical why not do a native UI?

Costa here, post author.

For the performance critical part, which is the screen-sharing and remote control, everything is done from Rust which is low level and gets the job done.

Native UI in this case wouldn't offer us much more. There is a case that maybe it could be faster in the video rendering in comparison from the browser's <video> tag, but we have not make any tests yet, maybe we should test this in the future.