I just noticed an interesting aspect about NVENC exporting:
When having both GPUs enabled (GTX 980 TI and iGPU in my i7 8700K) premiere used the iGPU for decoding and the GTX 980 TI for encoding. I expected this would speed everything up ... But it was rather slowing down!
iGPU + GTX: 155 FPS
GTX only: 250 FPS
So if you are having a similar setup try to disable the iGPU when exporting with NVENC. It might be faster!
Edit: It might be enough to disable Intel GPU decoding in the premiere preferences. No need to disbale it in the BIOS.