Having done some more testing on this, I think it definitely comes down to Javascript code (that of course runs locally) which deliberately slows down some operations, wasting CPU cycles along the way (so not even just clean "waits" that would just suspend execution for a little while.) Nice job. The good news with this is that if it's just JS code running locally, then it should not be too hard to find countermeasures. (And this would be explained by the fact that there is actually little they could do on their "end", as we had discussed in an earlier thread, so they basically have to resort to local "measures". At least, that's my conclusion this far.)
Firefox has a profiling feature, you can go to about:processes, and for a given tab, you can have it profile it for 5 s. The profiling is not ultra detailed, but it for sure shows that most of the CPU time is spent on JS execution.
Oh, and the JS scripts appear to be downloadable. I've just had a look at a couple - there is one which is called 'scheduler.js', which seemed promising. The source code of it is of course somewhat "obfuscated", but I'm sure some people will be happy to have a closer look. I'm not sure YT have been that smart with all this. Fun.