my RX 580 8GB Powercolor is underperforming it is getting 50 fps in fallout 4 in sanctuary and it should be getting 90 fps.i have a ryzen 5 1600 and it isnt running hot and same with the RX 580. i am running it at 720P and also the GPU usage drops down very low. I also get really bad performance in hearts of iron iv but i know it is really unoptimized.
Why should you be getting 90, what was previous card/system ?
Maybe you need to reconfig. ini tweaks/mods/settings to suit ?
You are right that you should be getting well over 50fps at that ungodly low resolution (why the heck are you running 1280x720 in 2018?) when it can average over 60 at 1920x1080. There are -a lot- of threads on multiple forums about low GPU usage, and one of the common themes is that they are using windowed mode instead of full screen which keeps the card in a low power mode. Make sure you are using full screen, and I'd suggest asking in either the developer's forums or the Steam forums.
If you installed the 'High res texture pack' dlc, that would be the cause.
Also as the game favoured nv cards, you need to make changes to suit amd card, as black_zion mentioned, you can get all the info needed for retweaking from steam and/or beth forums.
Edited a word.