|Description||I'm not entirely sure what's happening, but I seem to have a "pocket" of z-levels where if I'm somewhere in that pocket, my FPS drops considerably. If my camera is above or below it, the FPS is fine.|
One step above the pocket, z=76 gives me 100 FPS. Normal. (Not at all times, but always within a normal region of 90-100.)
Going down one z-level drops my FPS to 28. It remains in that region for quite some time as we descend, dropping down as low as 19. Ascending from the other side has very different effects: In z-levels in the 30s, my FPS is fine (90-100) but starting at the upper 30s the FPS drops slowly, with a major (about 30) drop at z=40, and getting to the lowest FPS of 20 around z=50, maintaining that level until z=76, where it returns to 100.
My only guess is that this region includes a dungeon feature I hadn't seen before, at least not in this situation. Even though I'm over 50 levels underground, I found a "passage descending downward" here that mostly exists in this pocket, which terminates in an underground ocean. Other than that, the levels are normal cavern system.
Even using a reveal utility indicates nothing that seems to explain it; the levels in question are mostly solid rock that opens into a cavern system with a lot of gold interspersed. (It also caused a demon release. Speaking of losing my FPS... Anyway, I backed up my save before Revealing for Science. Getting back on track now.)
P.S. Did my best on categorization. Couldn't decide between "map features," "dwarf mode - environment" or "technical - general."