You know the part where you go and get Roland? You also meet those Robots for the first time? Yea that big map. That map is where you stress PhysX (needs to be the first time you go through though since the Robots don't respawn the 2nd time through). Brings my 750 Ti from playing 90% of the game at above 60 FPS to playing it at like 15 FPS.
I think this is a driver problem, I had a 660 and the game used to run fine; but when I went to play it again about 6 months ago it was running much worse than I remember it (bloodshot stronghold 15fps when it used to be about 40) – I had updated the drivers since the last time I played it but not much else. This is on Win 7 not Windows 10, one guy said he gets the exact same problem on Linux
I now have a 1070 and the game still runs very jerkily; even if I do showfps it says the FPS are in the hundreds yet the game jerks and feels very laggy.
I think NVIDIA possibly introduced a bug in their physx implementation in their newer drivers; I might do a test on a spare PC with my old 660 and older drivers to see if I can find out what happens
Set your frames to unlimited, i did this and it drastically improved it, try disabling fxaa i think its called, im not sure why it just helped me. Maybe (im not much of a hardware guy) the parts dont have good synergy?