When I play Terraria, the game speed will change while I am playing. So sometimes it will be at "normal" speed, then it will dip into a slow speed, or a fast speed. It seems the actual game updates are slower too, not just a dip in FPS. For example, if I am mining I will collect more ore in the same amount of time if the game is sped up, and less if slower.
It doesn't seem to be controlled by anything going on in game, when there is a lot of things going on screen, it won't always go slower, and sometimes may go faster!
I have an AMD R9 290, intel i7 4770K, 16GB RAM. Running Win10 now, but it happened on Win8.1 too. This is in a single player game. There is nothing else going on in the background.
What might cause this? How do I prevent this?