I just saw post about that kid of issue at reddit.
One user wrote "
A year ago I was getting 400fps, now I'm getting 200fps. I don't think it's my machine that is failing as other games still rune fine and my system isn't even overclocked in the slightest.
I'll just copy & my response for him there:
I'm (for example) playing at toaster. Seriously: P2C 2,8 GHz, 2 GB Ram and Geforce 9500 GT. "Earlier earlier" I remember I was playing at ~"medium", and now I must play at "utlra low". But to be honest earlier i had old screen with 1024x1280 resolution. Now I play in full HD.
I also have antivirus (Kaspersky Free) and it's working somehow in background...
But LoL is still playable for me (but i don't have stable 60 FPS), it's not unplayable, like some people writing & I assume, that they have PC - not a toaster like me.
Maybe you can upgrade GPU drivers? Maybe you can upgrade CPU drivers (at least I know that kind of things exist)? Maybe you can upgrade Windows? Maybe you can upgrade other programs which works with LoL (I don't know: Java 8? Microsoft .NET Framework? Microsoft Visual C++?) I'm not saying, that this will help: I'm saying "maybe". Maybe you have some virus/trojan?
I don't know - I'm not a specialist. BUT I have all that things patched, scanned, seems clear, I'm even updating my "Ethernet Controller Driver" and... It's not a disaster with my toaster, like some people says...
But overall I'll admit - yeah, I also think, that there are probably some things to fix in client/game.