I recently built a PC and I've been having some FPS issues only in League of Legends.
My system is:
AMD Ryzen 5 1600 CPU
Corsair H100i GTX CPU Water Cooler
Asus ROG Crosshair VI Hero (Wifi AC) Motherboard
Gigabyte GTX 1080 Windforce OC 8GB GPU
Corsair Vengeance 16 GB DDR4
Corsair RM850 850 Watt PSU
Samsung 850 EVO 500GB SSD
Asus PB277Q 1440p 75HZ Monitor
I used to have a i7 4890k and GTX 1070, it performed well. But I needed more performance in other games so I built a new PC. I purchased a 1080 with the new Ryzen 5 1600 to try to get better frames in all games, I do get frames I should be getting in all the games I play (I think) except for League of Legends. League of Legends gets about 100 FPS when set to uncapped and goes as low as 40 when I get into big fights, I used to get 300 FPS and would maintain higher than 100 in big fights with my old PC. I get the same FPS no matter what my in game settings are (low quality or ultra quality.) So far, I've tried tweaking League in all sorts of ways to make it perform better but no change.