This is a bit of a long shot, but have you tried the “safe mode” on the various browsers, to prove that it isn’t some add-on that has slipped into all of them? I don’t know about the others, but on Firefox the --safe-mode command-line parameter will do it.
If that isn’t it, the problem might not be the browsers. The entire machine might be significantly slower than it used to be, and you’re only noticing it in the web browsers because they’re the heaviest applications you regularly run. You can try this to see if it helps, it fixes a lot of strange hardware behaviors.
If that doesn’t do it either, I’m sure someone else has other ideas to try.