I think time stretching is done natively by your browser, not by Youtube at all. I use https://github.com/igrigorik/videospeed on sites...allows any media to be stretched. Did you try another browser?
That's no excuse for YouTube because (a) audio processing can be done in JS/WASM and (b) they have the influence to improve browser playbackRate implementations to something better [1].
Besides, their Android and iOS apps do slow music as bad if not worse than on web.
If there's reason to believe this is a useful way to handle time stretching, then there's reason to believe the same browser could do it natively just fine.
Which one in that list is better than the default one in Chrome? What media serving sites that you know of intentionally don't use native browser APIs?
Besides, their Android and iOS apps do slow music as bad if not worse than on web.
[1] https://bungee.parabolaresearch.com/compare-audio-stretch-te...