I just realized something:
I always was complaining about the huge amounts of RAM Firefox and Chrome(ium) waste. I switched to Chrome at some point because it deallocates RAM more dynamically, making long-lasting sessions possible. I exploited this to a point where 60% of my ram was occupied by Chrome. Because Windows took care of the last 40%, my System became unusable when I opened more tabs.
This problem totally vanished when I doubled my RAM to 12GB. I mean, it was only like 20€. Maybe I should think more about my tendency to waste RAM.