I'm on a 970, and it definitely dropped below 30fps for me regularly. So much so that I had to turn off adaptive v-sync (I was getting serious screen tearing throughout). I'd make sure you don't have too much going on in your nvidia control panel that might be contributing to input lag. But I could never tell if the reason my dash chains never worked in boss fights was because of input lag or because of nerves...
Try turning adaptive vsync off and just using the "window max" mode in game. It gets rid of stutters or any weirdness.
How many frames are you dropping? The thing with frame rate drops in this game is that the entire game slows down too, so you'll notice you walk slower and input timings are slowed down... so it's not really input lag.