Due to the Problem of Frame drop and stopped sound on iMac

Iam playing the game now on

iMac (27-inch, Mid 2011) 3.1 GHz Intel Core i5 12GB 1333 MHz DDR3 ATI Radeon HD 4850 512 MB

But I can't play almost because of Frame drop and stopped sound in game after patching for the last version. I found the point of the error, that happens when champions cast some skill with the effect from arcade skins for example Mf's W, there appear some dots and get slow.

really it's impossible to play it now.... They need to fix the bug asap, I hope they need to care the client for Mac too.