I don't really know anything about FPS or computers but I don't understand why I can get 300 FPS in the range or in custom games by myself but when a game starts it drops to around 150-180 FPS. Why would playing with others affect my FPS?
beyond it being CPU related (which is largely is as others have pointed out) drop your graphics settings too tbh if you haven't already, i was playing on the highest for a long time and had the same thing but lower graphics settings actually equalised the FPS difference more and also got rid of lag spikes for me