Crimson 16.5.3 beta. R9 390 OC to 1118Mhz core. Memory standard 1500MHz No power OC.
I noticed a big frame rate drop when switching between full screen (60+ fps) and full screen windowed mode (~20fps) in Total War Warhammer. I checked and saw the same in Mordheim City of the Damned. When switching to full screen windowed the memory clock drops from 1500MHz to 150Mhz (idle).
Rolled back the driver and FPS was fine and saw no memory clock drop.