What are some of your forts that have had the worst FPS? It could be because of features, a single event that killed the game, or because it was such a big zone. Or whatever else.
0-1 FPS, 178 dorfs with a king. With 300 or so creatures, mostly in cages but things started to go downhill ever since I started to kill goblins with weapons traps and dwarves instead of caging them, making the mistake of giving them a chance to bleed. Now down to 0-1 FPS now during sieges, otherwise steady 1, occasionally rarely maybe 2. Game year 9-10. Game is going very nicely, but it takes IRL about one week to play a year, so not going there anymore.
1-3 FPS @ 60 dorf outfit, with largeish area but good perforamance until bloodshed happened and a couple years went by during which said blood spread around. Game year 4-5. There's some water flow in the area too and FPS has been breached, but this doesn't appear to have caused much problems performance wise as the same waterflows have been ongoing since the very start of the game so FPS hit hasn't been due to them. Again, slowdown seems to have been preceded by the first ichor/vomit/bloodshed that starts some out of control spiraling FPS loss to happen as contaminant spreading happens over a year or two and gets everywhere and then the FPS goes steadily down. I place the blame on any *named* contaminants that provide Genetic markers and information as to who bled or vomited them. Anonymizing bloodshed, ichors and vomits might help the FPS a bit, but getting rid of sticky and multiplying contaminants too altogether might be the smarter move and save lots of performance woes.
A few 2-5 FPS forts of 50-80 dorfs that I outright abandoned due to FPS going down to 2, which I blame on massive bloodshed. The sites not being very interesting to play, I abandoned.