Space.com has an article by Margaret Turnbull explaining why animals don't use chlorophyll. Essentially it boils down to "animals use energy faster than they can get it from the sun." This relies on three assumptions:
- The solar constant is going to be pretty similar for any planet capable of sustaining life
- You can't do better than chlorophyll's efficiency
- Animals will use energy at similar rates
The second assumption is probably pretty good. 8 percent is not bad. The first I don't know about, but for the sake of argument we'll accept it. The third is dubious. Why shouldn't animals go around slowly? There are well-known examples: sea anemones, for instance, or starfish, or sloths (though they probably go too fast).