I'm surprised AdBlock is so popular. I tried it a few years ago and noticed the sluggishness immediately. I know it's not a feasible alternative in everyone's opinion, but if you're running a low end machine and want to block the most pernicious ads (flash, multiple external javascript, etc.) then Firefox + NoScript is the way to go.
Presumably if you have a slower internet connection the performance cost of injecting stylesheets is offset by the reduction in how many images/flash banners/etc. you have to download.
I used to run a 700MHZ powerbook, and the fan used to whir away. One day I realised that having Firebug running wasn't helping! But I foolishly hadn't even considered it, as I was so used to running the extension. Adblock Plus I pretty much always install too, so would just think it was normal. It still feels much better than running the adverts!
I did try noscript for a while, but I got fed up having to turn it on for sites that just failed to work without JS, and got bored moaning about it.
Many of the sites with the most pernicious ads require JS to function, so NoScript isn't really that effective if you're going to, say, stream the latest episode of Game of Thrones.
That's why you only allow the CDN hosting the videos to run JavaScript and disallow the ad network hosts from running JavaScript. Best of both worlds - you get your video stream but never have to run the ad network JS.
