It's much faster than any Firefox fork mentioned above. There is Chromium 49 which is full-featured, supports all video codecs but doesn't support GPU hardware acceleration under XP.
I had the chance to try it and I wouldn't rely on it for everyday use. There is Advanced Chrome that has a build above version 51.0 but does not support hardware acceleration (yes it's possible under XP) and is not a real stable alternative because it crashes sometimes. So you know it's not that good on an old computer running XP. Since this is Firefox codebase, all of them are really slow because the unoptimized and bloated engine Firefox codebase generally comes with. On these browsers you must use an older version of uBlock, around version 1.13. These are Centaury (built from Basilisk), Mypal (built from Pale Moon) and New Moon (built from Pale Moon).
One of these is the Firefox branch, more exactly the forks that have been created from older versions of Firefox and the last versions of Firefox ESR that used to support XP.
OLDER VERSIONS OF FIREFOX FOR XP WERE FASTER WINDOWS
On Windows XP there are a limited set of browsers available. And especially that kind of people who need the capabilities of your great blocking extension the most. So that's not just a simple version number. At least twice as much than of Linux, according to NetMarketShare. Windows XP is still used by millions of people.The last version of Chrome available for Windows XP is Chrome/Chromium 49.So the minimum Chrome version number must go up to 51.0. I have already opened an issue in your global issue tracker but I'm afraid one of your volunteers has just labeled it as invalid, referring to another issue where it was stated that Chrome version must be bumped above 49 because of a JavaScript syntax it doesn't support. I have opened this here because this is only for you.