You might have heard of the Adblock extension that has a massive list of URL filters to block out adverts from most websites. Does this slow down Firefox, considering that for every little image and flash it has to consult that massive URL whitelist?
Most of the blocked content is made up of flash files, images and scripts. These are quite large and require time to download.
So, by blocking them Adblock actually improves the loading time, sometimes by a lot.
Yes, it does slow Fx down. It's a fact. For sure, you save the bandwith of non loaded graphics, banners etc. The negative aspect is, that ABP needs to check every URL (yes, resource locator) if it's on one blocking rule.
I recently installed one big big filterlist and it really slowed Fx down.
But: If you only use a small filter subscription, you can be sure there's no effect you'll notice.
It doesn't seem to slow Firefox down. If anything the impression I get is that sites load faster as they don't have to fetch so much data.
You could do some timings with Adblock turned on and off, but there are so many variables (caches, network latency etc.) that the numbers you get wouldn't be particularly useful.
Time for matching against a large list of items is generally not proportional to the size of the list (technical term is
O(n)). If it were, each Google query would take many years.
As others pointed out, the time saved by not fetching and displaying the blocked elements probably far exceeds time spend matching against the list.