Most of the time simulations crunch through really fast (in seconds), but quite often they get seriously stuck. It seems to happens especially with Treasure Grove matches, at best they will say Infinite Loop after several minutes and move on, but often it will just hang for 10+ minutes until I notice and reload tab.
Are these level particularly long to calculate?.. The difference in time (second to minutes) seems drastic.
It’s the simulation getting broken; there can be a few causes, but I’m guessing the ones you are seeing are probably this bug we see sometimes where NaN gets into the collision system and then propagates from there. Michael and I are going to try to fix a bunch of simulation bugs today so that it doesn’t get stuck.
Seems to work much better today. Although it tells less details now, so I might not be noticing (semi-)stuck ones. 
I didn’t fix anything yet, but I simulated all day on a few computers and then purged the queue of all the glitchy games that had built up. So it should take a while for it to start being glitchy again. We’ll work on mechanisms for automatically purging glitchy games.