I found a really cheap trick to help anyone who's been having FPS issues in League of Legends.
I've been told to alt-tab and close the LoL Client when in game. This works but it means I have to restart LoL after every game and can't see my stats. Then I heard that I should change the priority, but when I exit out of league and start it up again the priority is reset. So I changed the default priority.
Go to your Play League of Legends shortcut, right click on it, and click on Properties.
There should be a target box. Change this from
"C:\Riot Games\League of Legends\lol.launcher.exe"
to
C:\Windows\System32\cmd.exe /c start "LoLLauncher" /belownormal "C:\Riot Games\League of Legends\lol.launcher.exe"
Then click apply.
I know it sounds sketch because of references to system32, but this is what the new target link is doing.
C:\Windows\System32\cmd.exe: opens command prompt
/c: this is the close command once the task is finished, so cmd doesn't stay open once league is up.
start "LoLLauncher": starts the LoL launcher
/belownormal: sets the priority level of the launcher
"C:\Riot Games\League of Legends\lol.launcher.exe": Is the link to your LoL executable. You might have to change this if you installed League in a location other than default.
Your League of Legends game will now take priority over the client that's running in the background. You should find FPS gains of 20 to 30 frames.