I really can't explain why it could have happened, but once I went to Sancre Tor I took a HUGE persistent framerate hit. I've tried rearranging and disabling various combinations of mods, various in-game and hardware video settings, all to no avail...

Here's the thing: even with all my mods activated, with the same video settings from one test to the next, loading a savegame from before Sancre Tor has my typical framerate (40-50fps), while loading any of my post Sancre Tor games cuts to 3-6fps for interior or exterior cells.