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So recently I upgraded my old GPU from a GTX 660 to a GTX 1660 Ti (my AMD-6120 six core 3.5 GHz CPU is a bit old at this point but is still rock solid, I haven't seen any signs of potential bottlenecking) and one of the first games I opted to test was Mafia 2. I remember on the GTX 660 it ran smoothly with APEX PhysX turned off, but with it on it ran at sub 30 frequently. I remember looking up at the time if there was a way to get it to run better with PhysX on and I often saw people saying "you need a second card that is used specifically only for PhysX." I assumed this was just a normal thing in the early part of the 2010s, even the Batman: Arkham games released around then outright said "for PhysX you should use a separate Nividia GPU just for PhysX."

I wondered if with a new GPU that's many years ahead of what the 660 was capable of if the game could handle the PhysX stuff with just a single card, but nope it looks like performance is unchanged there. With PhysX on my FPS is stayed somewhere around 25-30 FPS, but when it's off I maintain a smooth 60 the whole time.

So yeah, does this 9 year old game still seriously want people to have two GPUs in their PC just to render some nice looking particle and cloth effects?
No, it is a case of the Physx version being old and super unoptimized, see Borderlands 2 for that.

At that time, having a second card to run those effects made it "easier", but still wasn't perfect.