I’ve noticed the performance dies even if you just let the game sit on the title screen. I start between 90 - 100 fps, and then if I wait 3-5 minutes it will go down to the mid 40’s. It’s not an instant drop, but you can watch the fps free fall over around 30 seconds and then never recover.
Playing the game I’m at 45-50 fps to start and then it drops to 17… so unfortunately I just can’t play the game right now.
Machine is just an i5-4670K @ 3.4Ghz / 16 GB ram / Geforce GTX 760
Hope that helps in some way.