I have searched on XDA for a fix that the Games are lagging on i9505 with CM or CM Based Rom's and i found the solution!
Thx to XDA Member nfsmw_gr who had similar problems on his S4. In his Thread he has made a script that will set your'e min power level of GPU to 0 (0 is Max 450mhz)
(This can cause overheating the Phone and more Battery drain.
That is Why he made 1 script to enable and 2 script to desable this mod.)
But i have looked in to the Touchwiz Rom and the Stock value in min power level there is 2 and in CM it is 3. So this means in CM the GPU can go deaper in the mhz and this caused the Game lag's.
I modified his script and make it work as an init.d script and now it will set the min level on Stock Samsung values (2)!
I have tried this and as a sample Asphalt 8 on High Graphic run's Smooth without Lag's as on the Touchwiz Rom.
Download the Script and flash it in your'e Recovery!
(This Script is for all I9505 Roms)