Because of the way VR renders to the headset, 1:1 resolution to headset pixel isn't 1:1. SteamVR has a way to modify it so you render say 1.5x or 2x the resolution of the headset to account for this discrepancy. If you're rendering at too high a resolution (the Index is higher resolution than the Vive), then your 980 Ti might not be able to keep up.
That's my assumption on that paddle game. You'd know for sure if you measured frame times.