GTA has been running pretty well since I got it, and now today, without changing any of the game settings, it's a lot slower. I have an RX 580 8GB and Ryzen 7 1800x@3.6
My friend had this issue what we did to solve it was we went into documents and rockstar games then into the GTA V folder and deleted the settings.xml file
sorry if this doesn't really help or if you've already done this but thats what me and my friends did.