My Graphics Card is on the compatibility List - but it is not used - it feels like the software ignores it.
Encoding is a CPU centric process in Premiere Pro and Adobe Media Encoder. The GPU does accelerate certain items in the overall export process, but not the encoding itself. The GPU will only accelerate the processing of GPU-accelerated effects.
Kulprit is correct. Look at the GPU accelerated list he linked to ... the GPU is used for Lumetri & other color items, Warp Stabilizer, significant frame-size changes, and some other things. If you don't have those going at the moment, that GPU sits there.
When you have a clip that has something from the list, it will suddenly jump to life. If the next clip doesn't, it goes back to near idle. And how 'hard' it works at any one moment is a result of how much work that GPU accelerated effect needs at that moment.
Exports and rendering are for the most part a CPU cores/threads/RAM issue. The more cores up to around 10, the closer to or above 4Ghz processor speed, the closer to 10GB of RAM per core, the faster your exports will go.