I ran tests on example.com, apple.com, and a WCPO news article (for the ad blocker tests).
The biggest performance issue is that extensions just dump large scripts into pages indiscriminately. Most of the time extensions don't do anything differently based on the content of the page.
I also tested a WCPO news article to see the impact of ad blockers. I picked a local news website specifically because they contain a lot of ads. If a page doesn't have any ads the performance impact of an ad blocker will be slightly negative.
What my tests don't pick up is extensions that only run a certain domains. For example, if you run Honey on a shop they support I expect the CPU consumption to increase a lot more.
I wrote a similar article last year, which tests a few other pages as well: https://www.debugbear.com/blog/measuring-the-performance-imp...
The boxplot looks encouraging (as in, the variance doesn't seem too relevant for drawing conclusions)
I hope to use this as another source whenever marketing wants to add yet another tracker.
Firefox for iOS is not really Firefox.
On mobile, with slower networks and much worse CPUs, uBlock often completely changes the experience. (thanks Mozilla!)
I would note though that only example.com was examined (and apple.com in one test).
I also did not see information if tests were repeated, as no variance/stddev is given. I'd expect it to be pretty high.