Hey thanks for the reply. I have tried lowest setting and it didn't make a lot of difference. The iMac which is older and has slower gpu and gpu memory is doing fine rendering at higher resolution. Could you be using GI on the LIght setup of the scene? There are couple of things that you could check such as GI(global illumination), camera rendering, update loop, or real light along with baked light. Run it through profiler and see where the gpu is hiccupping. Maybe there is something wrong with the shader?
I am a total noob and sticking to the desert and rock level for now so no worries :)
Cheers!