In my tests (albeit a few years ago), the actual text content on web pages was slowed down slightly by adblock plus. I measured with a video recording of the screen, and considered a page complete when the title and main body text of a page were loaded.
It became a dramatic slowdown with a slow CPU. I made a performance patch which saved 40% CPU time (by making a new css selector matcher). I even tried to submit the patch upstream, but my employer blocked it with bureaucracy.
