I've had similar bugs with Fortnite, in that the Vega 56 Graphics card only gave me about 20-30 FPS! This happened after a driver update. Solution? Reinstall driver with a factory reset and delete saved precepts, this fixed the issue (and also the black screen issue) and gave back the frames that was lost due to some conflict with the previous old and new driver update.
Whenever you have a problem, reinstall the drivers, not just a normal install, because bugs may not go away in this way, but you need to do a factory reset in the advanced option in the beginning of the driver installation.