Took a performance benchmark in both Firefox and Brave. Both of them are unusually slow (addons/extensions disabled, fresh profile), but Firefox especially so. Source for the huge render times seems to be desktop_polymer.js, specifically the part that's registering and setting up custom web components. Once the website loads, performance becomes a lot better.
My guess is that a Polymer update made Youtube slower for everyone, but SpiderMonkey isn't particularly great at the kind of excess operations that have been added. Firefox in particular seems to suffer from complete UI freezes whereas Chromium browsers seem to just have slow tabs when the browser is overwhelmed.
While I certainly wouldn't be surprised if this is part of an anti-adblocker mechanism, not every slowdown on Google's websites is done out of malice. Some of it is just caused by bugs.
My guess is that a Polymer update made Youtube slower for everyone, but SpiderMonkey isn't particularly great at the kind of excess operations that have been added. Firefox in particular seems to suffer from complete UI freezes whereas Chromium browsers seem to just have slow tabs when the browser is overwhelmed.
While I certainly wouldn't be surprised if this is part of an anti-adblocker mechanism, not every slowdown on Google's websites is done out of malice. Some of it is just caused by bugs.