Solved! Waffles had kindly changed the script that checks how many letters you've typed in the comments: now I don't have any lag any more (at least none out of the ordinary on this oldish PC). Bravo! Watch my typing.
Updated: here is a screencast of my typing
I am testing the speed of typing in this comment., on Firefox 5, on this Pentium IV @ ~2GHz, Windows 7. You can see how many letters vanish and how slow it is. Pay attention to the sound of my typing: I was finished typing half way through the recording, but the letters weren't quite there yet...
Updated again: Here is another screencast, with Firefox in safe mode (Help -> Restart with Add-ons Disabled), which disables all add-ons, settings, etc. Not much better. I also tried a virgin profile on FF 3.6 at the time (see below), just as bad. A mysterious issue.
When I type something in a comment on Superuser.com or English.stackexchange.com, typing lags badly. I see the letters slowly appearing one by one on the screen, sometimes two seconds or longer after I actually pressed the keys. Often it even fails to register key presses when I go too fast, or it wrongly puts the first letter after the second, even though I typed them in the right order. What can this be, and how could I fix it? It is driving me crazy.
This happens on those two sites, not on any other random forum or web form. I have typed in all kinds of different sites since I joined the SE/Superuser etc. family, but it has only been happening there. So I think it must be specific for the SE family.
It is very bad when typing comments, but usually much better in answers and questions.
When my text is auto-saved, I get a particularly horrible lag with missing and misplaced letters.
It happens on two different computers, both around 2 GHz single core (at least one has hyperthreading), but both with plenty of RAM, each with a different keyboard, monitor and all.
One PC is on XP Home, the other on Win 7 (fresh install), both with equally bad lag; but the lag does not happen, or is hardly noticeable, on my third PC with dual-core 3 GHz, 4 GB RAM, XP Pro SP3. All three PCs are 32-bit.
The bad lag happens on Firefox 3.6.13 and Internet Explorer 8.0. It is slightly noticeable on Chrome, but much, much better, not really a problem; for example, when I backspace a long word, it goes a bit slowly and choppy, but with normal typing it is hardly noticeable.
I have plenty of memory available when it happens, only 600 of 1500 MB are in use on this PC.
I see no spikes whatsoever when in happens, neither in CPU use nor in memory or disk usage.
I have shut down the only 3rd-party program that I had running on both PCs (StrokeIt, mouse gestures), but that didn't help at all.
I just noticed that the bug seems absent from a fourth PC, my old 1.5 GHz laptop with Win XP SP3 32-bit and 500 MB RAM. I have only tried it on Firefox there.
The slowness tends to get even worse as I have Firefox open for a longer time.