If you play in the adventure mode you may have noticed a horseplosion plaguing procedurally generated dwarf fortresses in the world, which drags FPS down to a crawl. Horses everywhere, horses on top of horses, with horses on top. Too. Many. Horses. And lag. To me this occured in every dwarf fortress my adventurer came across so far.
The catsplosion wiki is outdated and does not provide a working solution. But by trial and error I found a fix that works.
0. Please backup your saves before making changes.
1. For a brand new world, you can open a file called raw/objects/creature_domestic.txt and simply delete the horse entry. No horses in your world, easy-peasy. The entry in question starts with the line [CREATURE:HORSE] and ends with the line [TLCM_NOUN:eyes:PLURAL].
2. For an EXISTING world, this won't work, because the game will complain it's missing an entry for horse and won't load your save. Instead, open the same file inside the savegame file structure: data/save/regionX/raw/objects/creature_domestic.txt then find the same [CREATURE:HORSE] line and scroll to the eyes line: [TLCM_NOUN:eyes:PLURAL]. Insert a line after that and put in the following:
[SELECT_MATERIAL:ALL]
[IF_EXISTS_SET_HEATDAM_POINT:0]
[IF_EXISTS_SET_IGNITE_POINT:0]
[IF_EXISTS_SET_MELTING_POINT:0]
[IF_EXISTS_SET_BOILING_POINT:0]
This will make any horses you encounter in your travel simply evaporate out of existence. Literally 'poof!', harmlessly. Some horse owners might get upset over the loss of their pets, but I didn't see any fighting as a result, so that's a sacrifice I'm willing to make for my FPS gains. You can also remove this addition to the file later so you don't poof normal-sized packs of wild horses that you encounter in the wilds that don't affect framerate, and then add it again when you run into a problem area (all dwarf fortresses in my world seem to be affected).
Until Toady fixes the bug that causes the horse population explosion in question, this seems like a good temporary solution.