DPI doesn't really tell much (that only means something in hardcopy output),
it's the actual raster image size in pixels that counts.
Smaller pixel size helps, so does reducing the number of lights, the number
of reflective and transparent materials, reducing number of procedurals in a
material (marble, sandpaper, roughness, etc...).
But all that reduces the quality and "realness" of the image you are trying
to get.
So, in the end... faster and more processors - whether more physical
processors, or the single dual-core/quad-core kind - are all that really
helps reduce rendering times in any given image size... period.
