1) Before the problem occured, I could even watch videos on the second screen while playing. This makes me pretty sure that my PC is not too slow. But now, I have 100% CPU laod.
2) I mostly play with sound off (because of the video watching), but today I turned audio on. I noticed an audio delay of (estimated) 1/4 second. It already shows up at the start of the game, when the armorgames logo (with the two swords) is playing.