Weird Font Smoothing Issue
Well, to answer my own question: the problem was that my GPU was overheating.

I forgot to mention that this specific machine is a hackintosh. I don't know if such thing can happen to a real Mac, but anyway: here where I live it's pretty hot and dry, so there's a lot of dust accumulation inside and around my computer. The GPU cooler stopped because of this (last time I cleaned it was back in March), so I just opened up the computer case, cleaned it and now everything seems to be back to normal.