I've already googled "wordpress 2.5 slow" and seen a few comments.
We have two WP blogs on the same server at work. Our main one still uses 2.3.3, the new one for a client was created using 2.5.
They are running on linux, php5.2.4, mysql 5.
The 2.5 is incredibly slow - 20 seconds to load, consistently.
The old 2.3.3 version is a happy .3 seconds.
Plugins used are xml sitemaps, allinoneseo, akismet, sociable.
I tried disabling all plugins, to see if it made any difference. It didn't.
I added wp-cache, which helps for repeat viewing of pages, but not for new pages.
What on earth happened between 2.3.3 and 2.5 to make the page loading sooooo slow.
I'm too scared to convert any other blogs (our main work one, and my own very old one) to 2.5, unless this gets fixed.
Slow as a dog just doesn't cover it.
I will disable the auto-update checks, to speed up the admin interface, just so people don't give up on editing. But that doesn't help the casual visitor who is just browsing.