Hello. The issue I have is that almost always, every time I go to an exterior or a city interior from an interior, my FPS suddenly and dramatically drops, no matter that the game has only run for a few minutes. If I save my game then reload, the FPS return to normal, so I doubt it's something system-side (I've a Phenom II 2,8 Ghz, 4 GB RAM, and a Geforce GTX 560 Ti 1 GB, all drivers updated).

If I remember correctly, I had the problem even without using any mods (using 1.6 version here (+ USKP 1.05c), with a savegame that has always been 1.6), but I noticed that the issue is much more frequent now that it used to be when I began that new savegame. Bloating, maybe? (savegame size: 9,1 mb).

Any help would be appreciated. That bug/issue is terribly annoying.

See if alt tabbing through twice resolves this for you. Its a pretty common and known issue.