I used to be able to play the game fine for as long as I wanted, until a few days ago.
Now the game slows down immensely once in a while, for no reason that I can see. I can play fine for a few minutes, with great performance, and then all of a sudden it drops to single digit FPS.
If I switch to/from fullscreen mode, performance comes back to normal, until the issue happens again after a while. It doesn't seem to be regular intervals.
This of course makes the game unplayable. Sure, I can hit F11 twice and keep playing, but if the slowdown happens when a creeper is around…
I was playing a single-player world, but the same issue happens when I play on a server.
I did try the latest snapshot, and the exact same thing happens. I don't think it's my hardware, as the game plays very smoothly until the problems start. I am not aware of software changes since before I started having this issue.
I induced a crash with F3+c and included the report.