Left Steam on by accident when switching between Vermintide and Catalyst. Usually run with 100+ fps in Hyper, with no issues whatsoever, on my 1080. Experienced all kinds of hitching, and the game wouldn't settle until I turned off Steam.
Likely something to do with Steam overlay. Regardless, if you're one of these people that allows Steam to load with Windows, do the sensible thing before whining at DICE about performance, and ensure you're running a sterile desktop environment. It's not DICE's job to deal with Valve's coding.