I do wonder if their mobile version have a similar problem. I have noticed it chugs badly when opened for the first time in a while on Android, meaning i have to leave it sitting or a while so it can get things done before i can actually browse anything.
Thats the thing, it has nothing to do with page rendering.
It kicks in about 10-15 seconds after initial launch of the browser, and basically churns it to a near halt for at least 30 seconds before it becomes usable.
Could be that this tablet of mine provokes the behavior by having a slow emmc that is nearly full, but it has resulted in me minimizing my use of a browser i have been using for close to a decade across various computers and operating systems.
And no, it has nothing to do with extensions. I still see it with no extensions active. Heck, i even see it with guest mode active.