As the Time Capsule Gets Full, it becomes slower, as i think it scans all the data before you access etc.
This is probably the case -- Time Machine spends a lot of time starting and finishing the process. But as I asked above -- who cares, and why? It all happens with no user intervention, and no penalty that I can detect. So don't watch the grass grow, don't wait for the paint to dry, don't watch the pot come to a boil.