I don't know if it's Windows that's causing it, but the framerate seems about the same as it did with 8.1 except it's dropping a lot more now.
Action! settings:
I could usually have it set to High (which for some reason got reset from some update and I never noticed).
Specs:
CPU: Intel Core i5-3570
GPU: Sapphire Dual-X R9 280X
RAM: Corsair XMS3 12GB
OS: Windows 10 Pro 64bit
Action!: 1.25.6.0