So, as the title says. I play league from time to time with the wife and recently I noticed my fps dropping from 400+ to 90+ and everywhere in between. I've tried to cap my FPS at 120, helps with jerkiness of the ups and downs but it does not stay locked there. It'll go all the way down to 80+. I've reinstalled drivers, reinstalled the game and even did a reinstall of Windows... why not.
It could be caused by server lag or game optimisation if its not happening in any other games.
"Low FPS and lag sometimes show up at the same time, complicating things. For example, if you’re playing a massively multiplayer online RPG (MMORPG) and your character is standing in a major city with hundreds of other characters running around on your screen, you may experience both low FPS and lag. The lag occurs because the server is struggling to keep up and slowing down with so much happening in one place at once, while the low FPS is a result of your computer struggling to draw all those characters on your screen at once.
In this case, low FPS and lag have the same cause – more people than the game can handle in one place at the same time – which may confuse some gamers into thinking they’re the same thing. But they’re not – the large number of players is causing both performance problems on your computer (low FPS) and network issues (lag)."
Alright, try disabling vsync and fps cap in league.
Go into Nvidia control panel and force league of legends on the program profile to use maximum performance. I suspect its just going into a lower power state due to the low usage league of legends has.