Nexus 5 Gaming

Hi all, been gaming on my Nexus 5 pretty frequently lately and I have some concerns. I have read some threads about how great games like Dead Trigger 2 look and run but I can't get mine to run stable on medium settings! Is this naturel, because I know it's got a Snapdragon 800 as it's processor and that's a pretty solid setup. I know the processor is aging and so is the phone but shouldn't I be able to run aging games at pretty great FPS'? The rom I am running is a more recent version of Cyanogenmod 12.1 CAF and also use greenify to keep apps hibernated . Also keep in mind I tested this on stock 5.0.1 and 5.1.1 and still the same results. Thanks!