i5-3570k, GTX 980, 16GB RAM, SSD. Windows 10.
Game which was running perfectly at high FPS is now extremely slow even in menus. Music appears to be slowed as well. CPU utilisation still below 20%. Only thing that alleviates the issues is holding and moving around a Naga spellcraft card which restores game to my usual smooth speed/FPS. Casting or letting go of the spell makes the game immediately go back to slow motion. There is also about a 5 second delay on pressing buttons like the tavern refresh button due to this slowdown.