Yeah, same problem here. I guess last patch or the patch before the last one caused that my game gets also lagged a lot. As soon as I start a game, all sounds in game are off and every 3rd second my FPS drops from 120-140 to 5-15... Terrible, game is unplayable coz of this bug. After I experienced this problem few times, I've also tried every possible solution (reinstalling the game, repairing the game, launching the game as administrator, cleaning my PC and so on), I've found out one solution, but it is not permanent and you have to repeat it every game. The solution is, that as soon as you launch a game and get connected to it, you have to disconnect the game (go AFK) and also close your client. Than you have to launch the client and log in again. You shouldn't be hearing any sounds during logging in. As soon as you will log in, your game should start immediately without any manual reconnection needed. As soon as you will be in a loading screen, you should hear sounds from client after loggin in. Than (if your game will launch, coz it crashed few times and I wasn't able to reconnect again) you should hear sounds in game again without any problems and your FPS should be okay, at least for the game you're actually playing. After the match, you will leave your premade group and after launching another game, you have to repeat it from the beginning. (PS: Since last little update in LoL, voice chat doesn't work anymore after proceeding this... Strange and anoying at all.)
I hope it helped you a little bit, this is a solution which works at least on my PC. If you have any better solution, please let me know, coz this one is slow, risky and annoying. RIOT should solve it soon...