Since you're talking about animation export, I did a small test yesterday with Pibuz's warehouse. I made this video to answer a question about post-production motion blur in another forum, but I got an interesting result in render times:
[flash=640,505:1e3wj8kn]http://www.youtube.com/v/iNVqKh2k164[/flash:1e3wj8kn]
The reason for this seemingly absurd result (Twilight being almost as fast as SketchUp) is the same you noticed: During animation export in SketchUp, CPU use was just 25%. This is because the Core i5 has 4 physical cores. Who knows, perhaps when desktop CPUs with 8 physical cores arrive, we'll see biased raytracers become faster than SketchUp's native renderer because of its lack of multi-core support. Please note that I'm not complaining about SketchUp; as has been already mentioned, it's a modeler, and has never been marketed as a render solution.
BTW, in the Twilight test I used an HDR as the sole light source, that's why you don't see hard shadows. Of course, you can use both HDR and Sun at the same time in Twilight. The variation in frame height was my mistake, before you ask.