Sadly nothing, I see.
I too have the same problem. I never finished PoE, completing the main story up to the point of no return, but then heading off to do the White March Part 1. Even then I was getting long load/transition times (long enough that I was surfing the web on my laptop while waiting for my PC to load the next area), and was dreading anything the required me to go into houses (or god forbid, to a house's upper floor - aarrgh!). Eventually, after a relatively short amount of time in part 1, I just gave up, bought another game on steam and archived PoE.
With Part 2 out I thought "maybe they've attacked this issue" and have given it another brief go. Transition times are about 25 seconds (I timed them this time) which feels better than I remember, but is still shocking. Unsurprisingly, all the activity appears to be HDD related (CPU barely breaks 15% usage, memory usage is only @ 6gb for the whole system out of 12gb). Guess an SSD would help, but really, it's daft that I'd have to upgrade my SSD just to play this one game.