I've been having some issues lately with Premiere Pro and Media Encoder not using CPU properly.
I'm having this issue on 2 macOS installs, Sierra 10.12.3 and High Sierra 10.13.6, the latter is a brand new install I did today.
This issue is happening on all Premiere projects I'm currently working on.
It usually goes like this, I send it to Media Encoder, hit render and it goes smootly, then, a few moments later, I send a new version to Media Encoder, hit render again and render time is 20 times more than the first time. On the last project it was 3 min vs 60 min.
When it goes smootly, Media Encoder reports normal CPU usage, when it is slow, it shows around 20~25% of usage.
Sometimes it works again after reopening Premiere and Media Encoder, sometimes only after a restart.
I also have the same issues when trying to render in and out inside Premiere.
I'm not doing anything fancy, 4K ProRes footage, Lumetri Color and a few imported titles created on After Effects.
This started happening after upgrading to CC 2019. I don't have this issue with AE, just PP and AME.
Intel Core i7 7700
NVIDIA GTX 970
250GB Samsung 860 EVO - System
240GB Kingston UV400 - Cache
1TB Seagate Barracuda 7200RPM - Project Files
8TB Seagate Barracuda 7200RPM - Footage
macOS High Sierra 10.13.6 - Hackintosh, brand new install.