Decal performance does not scale with GPU speed

I’m working on a VR project that uses deferred decals. When profiling the game with both a GTX 970 and a Titan X, I see roughly a 50% speed increase in everything except PostProcessDeferredDecals, which take up 2.5 ms of frame time on both cards.

Is this normal? Why would deferred decals not render faster on a Titan X than a 970?