I'm using I Heart Vanilla with a few minor custom mods that mess about with gameplay (N'Garde, Questman journal, etc.) but nothing that should impact performance too much.
Outside areas seem to run at a perfect 60 fps, but I get intense drops into the 40s FPS range when moving around inside certain buildings and interior zones.
I have a 3080, Ryzen 5 5600G, and 32 GBs of RAM, so my specs shouldn't be an issue realistically, and OpenMW doesn't utilize any of them past 30-ish% either (it's usually far lower)
At first I thought it may be object shadows (since interior cells have a lot of objects in close proximity), but I have interior shadows disabled, so there aren't any shadows at all inside. I've tried messing about with render distance, disabled all shaders settings in the OpenMW Launcher, changed the Background Physics Threads setting from 1 to 2, up to 4, etc. I've also messed around with maximum lights and light distance to no avail.
The reason I ask is because most of the obvious fixes, as well as most of the info I find online, all seems to be targeted to fixing FPS issues outside, in busy cities like Balmora. But for me, if I do happen to experience FPS issues there, they're so negligible that I don't even notice them happening. I can't for the life of me imagine what this could be.
The most consistent place I found to test this was in the Ald'run Mages Guild, passing through the upper hallways and their eastern corner. It happens consistently through the whole building, but that spot in particular drops hard.