I am wondering the same wince I have a Ryzen 1600 Overclocked to 3.6Ghz with 16GB (2x8GB) DDR4 and an XFX RX 580 GTR. Average FPS are 35-52 (Sometimes 74 while looking to the sky). I think it's a driver optimization since the consoles have an APU (AMD based) and are more consistent with framerate there. Also remember Hello Games is improving the looks of the game with every update, so it does impact the graphic performance.
AMD please work on optimizing this game!