TL;DR Get off Adblock Plus and onto uBlock Origin.

ABP has that stinky practice of marking ads as "acceptable" (basically those companies pay ABP to be on their "allowlist").

As has already been mentioned - stop using ABP, and use uBO instead. It's a lifesaver.

Google slows down Firefox. After I installed UA switcher and changed UA for YouTube it became very snappy.

Everyone keeps pointing the fingers at ad blockers, but it's general enshitification. A website which does nothing more than display some thumbnails, some text comments, and a video should not work as poorly as YouTube does. And yet, they are so many layers of recommendation engines deep that blackbox AIs have more control over the website than actual humans, and more CPU cycles are spent guessing what I want to watch than loading what I am currently watching. I have the most expensive consumer GPU that AMD makes and I still regularly see graphical glitches on YouTube

How do they explain the lag other places outside their Official App on mobile?

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I had forgotten about this so evidently it has stopped, that said I have only ever used ublock origin and it was happening to me, with that on Firefox so I don't know about this theory that it's just that one particular adblocker.

I find it hard to let go of the idea that Google was doing this, but then again I suppose the fact that it isn't now would suggest they weren't behind it in the first place since the supposed motive for it was to push people to Chrome and if you just stopped doing this after like a month tops then it wouldn't be a particularly effective strategy.