What do you mean by working better with speed controls? The playback speed multiplier affects both the video and captions for me on Youtube. I like opening up the "transcript" too from the hamburger menu on YT.

For local playback of media files, I use daum kakao Potplayer, I have never come across something more power user friendly. MPV is weaker in a lot of areas, in my opinion. Potplayer interfaces with everything I'd want including vaporsynth and madvr and it has options for subtitle resync based on a multiplier and also converting based on framerate 60 -> 30. There is even an option for live subtitle translations. I personally like using it with smooth video project, but I understand it's not everyone's taste.