I haven't seen significant problems from javascript being too slow on things that use web MIDI. Here is a web based "piano karaoke" app that runs quite well in browsers that support MIDI (and since it syncs with YouTube videos, it really kind of has to run in a browser vs native)

https://www.youtube.com/watch?v=NV6rdmdZnkA

Get in touch if you want to beta test it and let me know if you think it is fast enough on your set up, etc. Hoping to put it on "Show HN" soon. rjbrown at gmail More about it at https://pianop.ly/portfolio/