We have 2 WP-blogs installed in our server:
The first one with 2.5.1 version and the second with 2.7.1 (with different databases)
The first one, with a lot of post, loads inmediately and works fine. The second one needs from 10 to 20 seconds to load, in the main page and in control panel too.
Something rare: If you click twice it loads fast.