I see the problem, but I don't see a clear analysis on the actual source of the problem. I assumed the issue was mainly single core performance, but he is also suggesting context switches could be the cause?

So could you fix that with a new scheduler? Or you just need another SoC with better single core performance? I could imagine that the latter already exists, just not in soc with >16 cores. My naive view is that such high core count system comes with tradoff on core size and interconnect/memeory bus complexity.

And I mean.. my phone is a middle lower end device and for sure I can play youtube videos (maybe in a popup as well) and run the browser without noticing that much difference from my laptop.

I think the single core performance would be bearable if it wasn't combined with maintaining a custom built kernel.

I don't think youtube playback is a relevant comparison since it uses ~0 CPU. Pretty much all phones have hardware accelerated decoding. Lots of TVs and streaming devices use an ancient Android phone SoC yet they too can play YT and run a browser. The entire UI is often a local web app.