said by Oedipus:
I've never seen it do a startup scan, either.
It happens in background. The disk churns and delays the startup of other items, slowing the overall process and delaying the time the desktop is fully loaded. On older machines this will be more pronounced. Read the entire story I linked to for more pro/con arguments about it, and confirmation that it does happen.
I did not just hear about it. I noticed it for myself when I recently started using the product on several machines and then went looking for a solution.