While size would matter, I don't see why metabolic rate should have anything to do with it. It's also funny to hear it described as 'time going slower.'
The nervous system pathways for flies are much shorter. Therefore, flies have lower lag. Go figure.
Just like an L2 cache on a computer processor, since the speed of information travel is pretty well fixed for the selected technology, using shorter path lengths yields faster response times provided the tasks are simple enough to benefit from it. Reflexes in people are like this. http://en.wikipedia.org/wiki/Reflex Detailed thinking has a lot more overhead. And while stimulus-response is more rapid for reflex type behaviors, the speed of thought is the same. (An L2 cache won't change the speed of light.) Using reflex is just a more efficient arrangement for certain types of tasks.