To update - I didn't reinstall, but by forcing 32 bit realised the slowdown/memory leak/crash appears to be caused by the IPA. When I used a vanilla .exe rather than the IPA'd one, it works fine.
I've just reverted the IPA changes and will leave it at that for now.
Thanks for your suggestions all!
I downloaded the all-in-one repack and just dragged it across - I figured it worked fine since the game would start and it does technically play. I don't think the issue is that the game runs worse, as such - it seems to accrue memory until it pretty much just overloads and dies. I assume a...
So I just started messing around with this game and I'm getting horrendous memory issues the second I hit anything involving the actual engine (like, task manager shows the process hitting around 14,000,000 K followed by the game dropping to 0.2 FPS). Machine should be able to handle it fine...