How are you exporting?
Are you doing a direct export from Premiere? Or are you cueing the job through AME? (Adobe Media Encoder)
There definitely does appear to be something wrong. Here are a few things to check.
With Direct Export, your GPU and the RED Rocket should both be doing most of the heavy lifting. I see 5% CPU usage in this scenario when downscaling from 5k HD, the GPU helps a LOT. The Rocket frees up CPU cycles because it's doing all the decoding.
If you're cueing through AME, there is a chance that it can't access the Rocket if PR is still open. When I cue through AME, I always close PR first so that AME will have access to the Rocket card.
As I've mentioned before, I think that the issues here could be related to you using multiple GPUs.