Cannot Use Other Programs when Media Encoder is Encoding on Mac OS Catalina

I used to be able to leave Media Encoder running in the background whilst continuing working in After Effects or Premiere Pro. Now media encoder takes over the entire CPU and I can't even use a browser or edit a document if it is encoding. 

I am presuming this is something to do with CPU priority. I have tried using the 'renice' command in terminal but it has no impact.

I have a feeling it is a fault in Mac OS rather than Media Encoder as the whole ecosystem has changed with Catalina. Has anyone else experienced this issue? Thank you. Tim