I am 70 hours in and just encountered the same issues. Long loading time between areas and cutscenes. Floor textures either missing or appearing as water, and if you step on those areas you fall through the floor forever. This applies to the companions and not just you. The Necropolis Halls are now unplayable. I'm trying to do an Emmerich quest where I'm supposed to follow him, but then he gets caught in those floor gaps and can't get out, stopping the quest from progressing. I've restarted the Xbox, quit and restarted the game, and tried going to earlier saves, but nothing helps.
I should also add all this happened for me after Patch 3. It is not a corrupted save. I've gone back to saves from before I saw this issue and all of them are affected. It's a general bug.
