EDIT: The issue MIGHT be that osu isn't actually 100% fullscreen somehow, I had skype messages pop up while playing and I can somehow make puush screenshots while being fullscreen which isn't possible? (It MIGHT be windows ink related since it apparently reactivated itself)
Editeditedit: Solved it by turning on compatibilty mode, thanks to a tip from Nathanael.
Since a few days ago I have input lag with my tablet as if I'm playing windowed mode, even though I'm in fullscreen, I think it might have to do with a windows update but don't quote me on that, at first I thought it had to do with windows ink but i completely disabled that and restarted my pc, didn't change anything. I'm using windows 10 and my tablet is a wacom bamboo ctl 470/k
So far I tried reinstalling the drivers and doing a system restore (even though that was just to yesterday cause that was the only restore point available)
osu! version: 20160821.1 (latest)