Exactly the same issue just this morning (was fine last night).
i7 6700k , Gigabyte GAMING z170 mb with 16gb corsair DDR4 RAM, Gigabyte GAMING GTX 1080, LG 34" ultra wide monitor on Windows 10 64 bit with the creators update.
Last night I was getting a rock steady 60fps with all settings on max at 3440x1440, this morning I cant get over 20fps on low settings at 1920x1080.
I have checked World of Tanks this morning (120fps on max at 3440x1440) and Shadows of Mordor (75fps at 3440x1440 ultra settings) so my PC is fine
If I run Windows Resource Monitor and MSI Afterburner while in WoWs my CPU is at 80% and my GPU at 8% so for whatever reason WoWs is NOT utilising my GTX1080 at all all of a sudden! It doesnt appear to be even using DirectX since the Riva Stats for MSI don't display in game
What is the problem WG? Right now my 30 days premium is ticking down and the game is unusable