Monitor your GPU with something like EVGA PrecisionX, or M-MSI Afterburner. My Macbook has a GT750m that would idle as Intel HD 4xxx (don't remember the exact model) and for the life of me, I could never get my vidya card to automatically switch to the 750m when I played this game. It's not just an lolmac issue- my friend uses a Lenovo laptop with a GTX 660m, and just realized this today lol.
¯\_(ツ)_/¯ (Thanks Korea!)
Assuming that you are not using an external hardware adapter like the vidock, that GT750 SLI is probably the mobile version and as such, uses nVidia's Optimus technology to save power. However, it is possible to disable that through the power settings on your icon tray, preferably under the plugged-in settings since you wouldn't want your laptop running on discrete graphics 100% of the time sans battery. xD
Also: This game is also a programming mess and you probably can never achieve stable fps in high body count places even with a high-end PC.
Edited by kimsera, 12 August 2015 - 09:50 PM.