Tor program constantly shifting "Identity" connection while browsing, you can also right-click on the "Vidalia" program (which comes with tor) and select "New Identity" which often helps ..
Because tor client uses such a complex set of networks, so this will help, because there is often some "slow" servers to go thrue, when switching "identity" will some of these be replaced, "Tor" do this it self, but for speed up, you can tell "Vidalia" to do "New Identity"