In the picture below, you can see that the H264 maxes out my CPU at 92% while I transcode a 2.7K GoPro video. Also, final FPS of the encode is 23:
Now here is the encode using the Maxwell class CPU on the GTX 970. CPU utilization is down to 15% and my FPS goes from 20 to 38! Impressive!
The params are be a bit different between the two, but I think they might be good enough for a comparison.
ffmpeg -i GOPR0099.MP4 -profile:v high -preset medium -vcodec libx264 -acodec copy output2.mp4
ffmpeg -i GOPR0099.MP4 -c:v nvenc -acodec copy nvenc.mp4