well i normally play at 60 fps every game but for some reason the last game i play was 15 fps for some reason this is the first time it has went that low i normally play at all medium settings and still have 60 fps but in this game even when i set all my settings to low i only got up to 20 fps and this lag is only ingame my computer is acts like it normally would other than that if you want more information please ask i just want to fix this

1. Do a malware scan
2. Play a bot game and see if the problem still exists.
3. Clean your video card with compressed air, some underclock if they overheat.

Those are probably the two most common problems.

Edit: also, uninstall pando media booster if you installed it with LoL.