According to me it's not a problem many people having the same thing me too
So I don't remember where but it was in LOL's site that they tell you it's fine and I'll tell you something maybe it's true or maybe not
It's basically when you on loading screen there is not much to run that's why you get 300FPS but when you play there are too much things to run and it will be normal FPS s in any game right?
Examples
If I I play Lol I'll get on loading 200FPS and when I play I'll get 64_40 but to any other game I'll get 64-40 so my normal FPS is 64-40