The dots at the edges are moving faster, so when you can see only them, it looks faster. Try not blocking it and just focusing on the edges, you'll get the same effect (although slightly weaker).

It's kind of like when you're in a car - look straight ahead and it seems to be going at a reasonable speed, but if you look straight to the side, it seems a lot faster.

this

The illusion of perspective is broken when you don't see the center.

theory of relativity and time bending