60fps H.264 encodes are very slow, use limited resoruces

I'm encoding a few different pro-res multi-camera streams into an h.264 from premiere. It can playback in premiere at 200-300fps, but when I go to encode the video it renders it slower than real-time and uses only 10% of my CPU/GPU. I'm reading all the files off of a very fast (5GBS) SSD so its not disk that is the bottle neck. It seems like premiere's h264 encoder really struggles with 60fps for some reason.