what do you mean by lag. its a term that is meant to mean a gap between input and action on screen, but is often meant to mean (badly) that the frames per second are not very high. The former can be caused by poor internet ping times in online gaming, or poor cpu/gpu/software setup. The latter would typically only be caused by a poor GPU.
With regards to the question of a bottleneck, there is always a bottleneck, always. The question is really is there an inbalance between cpu - gpu - memory,and in yoiur case i'd say there is no significant difference in capabilities of the cpu/gpu/memory.
Are you on latest drivers? do you have folding at home, seti or other cpu intensive apps open in the background?