Seems to have fixed itself. I let it run for about 45 mins at 1 FPS in the hopes of making it to an island so I could throw the raft on my lift. Just before I got to the island, I jumped back up to 80 FPS. Still not sure what happened. I hope there isn't a 'death zone' on my map with too many free-floating items. I know the only other time I've seen this happen in the hundreds of regular SM hours I have is when we accidentally created a few hundred unwelded blocks on the ground. Had to pick most of them up then, but without a known cause in this case, I'm worried it'll happen again.