I've been having this.
My videos are also randomly going from 4k to 360p, and having long hitches/pauses. Thats assuming the page loads at all, which may take a few refreshes...and its not an internet/connection related issue.
happening with Firefox with ublock origin.
edit Honestly, the youtube pages (and only the youtube pages) act slow and laggy like the CPU is running at 100% doing hard number crunching. yet system monitor shows its not even breaking 10% load on the CPU, and that firefox's process isnt even using half that, so it feels like theres something artificially slowing the page down, in retrospect.