Same for me. Since last week's patch, my usually 90 to 100-ish fps now drops drastically in the middle of games (no matter which mode or map), usually crashing as low as 20 fps or sticking around 30 as soon as teamfights start or there are many spells being cast. Fps that low are honestly unplayable with. I can kiss all my skillshots goodbye while it looks like i am playing with rubber bands attached to my champion.
By now I am assuming it got something to do with certain skin models or particles since this happens in about 75% of all games but not in literally every single one for me.
And yes, I tried all the troubleshooting crap, reinstalled the game, followed several "how to increase your fps"-guides and ran the hextech repair tool, ran corrupt file detection, updated drivers... nothing helped. Lowering the graphic settings doesn't change anything either and why would I have to do that if my graphic card has been totally fine with League ever since I bought it about two years or so ago? So yes, this is a bug and it is super annoying. Please fix this and keep this thread bumped, a friend of mine got the same problem and he is on a completely different system, not even using Windows and still: Same fps drops since last patch, not always but most of the time.