Bootcamp Choppy Gaming

Just installed Windows 10 and went through all apple and windows updates and I'm getting huge choppiness when trying to play games from Steam, specifically Assetto Corsa Competizione. Everything else with Windows is super fast and snappy, but once I try to play the game I get insane lag and can't even navigate through the menus. I've even tried changing game settings to the bare minimum.

I've got to be missing something, can someone help?

I'm on 2017 5k iMac with 32GB of RAM and a 250GB internal SSD. I did choose the minimum 64 GB of bootcamp partition, but I'm running games off an external SSD plugged in through USB. Could this possibly be the issue?

Edit: the game seems to launch ok in minimized view but becomes extremely laggy once I go to full screen. I'm sure this is a basic setting I'm just overlooking. Any help is appreciated.