I just recently installed the this latest version of Wordpress (2.5.1) and quickly found it hanging (while trying to load a page) for as long as 40-50 seconds! I then proceeded to isolate the potential causes (database, custom fields, custom theme) and have since found out that something stinks in 2.5.1's core.

He details some of the steps he took to try to track down the bug - installing on another server, searching the web for others with the same issue. His suggestion?

Before you upgrade your old working installation of Wordpress, I would suggest that you test 2.5.1 in some other directory just to be sure it works with your server.