not sure where the 'lighter resources' thing is coming from, unless its due to slightly better library/workspace responsiveness due to a CR2 with far fewer morphs in it, or one less skin texture.

V3 is 75k polys - actually much higher poly than V4 (55k).

its likely due to the fact that nothing out of the box from that era had advanced shaders or lighting - just a tex map plugged into the diffuse and perhaps a bump map.

you can load modern shaders onto the gen-3 figures but then rendertimes actually match those of Gen4.