Im not sure if its only me but Media Encoder is been rather slow when it comes to Gpu acelerated rendering. Usually when i check its my cpu doing most of the work while gpu is idleing and all. Im not sure if this is a proof but ill do more test. But what im sure rendering takes longer these days and i usually have my projects gpu acelerated
Sorry for the trouble. Ideally, GPU acceleration is used if you have used any effects or plugins which graphical processing, however, I totally understand your concern that you wish to get faster performance from the software. The performance is dependent on the system hardware, media used for editing the complexity of the project. We'll need more information to refine the actual reason for the problem here:
Are you using AME with After Effects or Premiere Pro?
What are your computer specifications(OS, RAM, GPU etc)
Do your slower render time with all the files/projects?