When my HDD is being accessed (a different one from the one FF14 is on, Final Fantasy is on my system SSD), my mouse input is stuttering. It will hitch and stop for short moments while the HDD light is flashing.
Gamepad input is not affected, also input in Windows itself is fine.
This started happening with one of the recent Windows updates (Win10 Pro 1709) and is only reproducable in Final Fantasy.
Changing fullscreen/window options won't help, same for software cursor. The Framerate remains solid and high during these hitches, also lowering the graphics options will not affect the problem in any way.