Oh. My. God.
I've just installed my new PC. 64bit and 32gb of RAM.
My browser issues are gone - almost everything loads in under a second, even huge folders barely exceed one second. I don't know what did it - the RAM or 64bit or something else - but after a decade of pulling my hair out, I can finally float away in an uninterrupted workflow!
Though I'm still a bit scared that the problem might reappear... *knocks on wood*