Act 1 and at the first half of Act 2 ran pretty well for me, generally 80ish fps on max settings everywhere (6700xt @ 1440p). It’s after that when it started running like a three legged mare; frequent random slowdowns to about 15fps. I suspect that they got the game mostly finished and then started their optimization pass at the beginning, and just hadn’t got to the end of the game by the release date, which was moved forward last-minute to avoid Starfield.
The end of the game doesn’t look any more complicated than the beginning; suspect they just ran out of polishing time.