I never used DS3. I actually started with DS4.5, but in my experience there has been a significant reduction in render time in DS between DS4.5 and the latest release of DS4.6. I think most of this improvement has come from newer versions of 3Delight that are packaged with the latest releases of DS.  The bulk of this improvement was maybe a year ago or so. I remember re-rendering some big scenes and noting a significant reduction.

The AoA Advanced Ambient light can also provide environment lighting with occlusion and shorter render times than than the UberEnv lights. The AoA advanced spot and distant lights can speed up renders and give you more control over how surfaces are lighted.  The AoA lights are not part of DS. They are separate products that you have to buy, but I believe they are only available for DS4.6+. The AoA lights have a bit of learning curve. They are more complicated to use (well, maybe not more complicated than UberEnv2).

Even if you are not using any Genesis or Genesis 2 figures, you can take advantage of these improvements.