You are asking a very relevant question. I believe nvidia analyse the game with our graphics cards and find the best setting for performance and quality. I clicked optimise and it change the graphics settings. Its generally reasonable performance with the nvidia optimised setting but it can get laggy in places. There are too many settings to tweak in this game though
I clicked optimise for "Quality" over "Performance" and it set me VRAM usage to 3582, when I only have 3012.
Optimised my arse.