Always this. If are running 2 monitors with windows mode you'll get janky ass phantom "alt-tabs" where you're still in game with mega FPS drops
Do you mean that when you change focus from the game to another window, the frame rate tanks?
If so, then that is by design.
You can fix that by adding MillisecondsToSleepWhileSuspended=0
to the [General]
section of your game.cfg file (found in League of Legends/Config/game.cfg
)