AdBlock Plus is the most popular extension for Mozilla Firefox and Google Chrome. But its ad-blocking benefits come with a price: very high CPU and memory usage, which could be a big problem if you're surfing around on a computer without high-end resources.
Mozilla developer Nicolas Nethercote explains why ABP is so CPU- and memory-intensive:
Second, there’s an overhead of about 4 MiB per iframe, which is mostly due to ABP injecting a giant stylesheet into every iframe. Many pages have multiple iframes, so this can add up quickly. For example, if I load TechCrunch and roll over the social buttons on every story (thus triggering the loading of lots of extra JS code), without ABP, Firefox uses about 194 MiB of physical memory. With ABP, that number more than doubles, to 417 MiB. This is despite the fact that ABP prevents some page elements (ads!) from being loaded.
A Chrome developer going by the handle "Klathmon" also suggests on Reddit that ad-blockers all inherently use up a lot of memory, and AdBlock acknowledges in this post that this memory consumption is a problem.
Chrome likes to crash on me every now and then, and sometimes I have to turn ABP off if the page has trouble loading or isn't formatted correctly. Now I'm considering removing the add-on. For some, the performance hit is worth the privacy protection and never seeing obtrusive ads again, but I'm a heavy tabs user and would rather see ads than have to restart Chrome all the time. Hopefully, though, with ABP aware of the issue, they might be able to fix it.