I'm having strange performance problems on my MU install of 3.0. I've been testing for several weeks, and here is my findings:
* When a new site is created, it runs fine performance wise. I'm convinced that this, plus other tests, indicates that the server itself is not the problem. My instance of MySQL is running on localhost to the server.
* After some time, the site will begin to slow down, particularly on the admin side
* This performance problem persists, despite using the TwentyTen theme and disabling all plugins. I paid particular attention to the domain mapping plugin, which I believed was the culprit, but turns out was not.
* Apache logs look good. I had some problems with the sociable plugin, which I've determined is not 3.0 compatible. I've uninstalled it.
* The root domain blog also suffers from the performance problems, even though it is the least configured than all the sub-sites. The root blog is running TwentyTen and has very few plugins, all of which I disabled at one time to debug.
* Potentially related, but doubtful, the popular Maintenance mode plugin, when enabled, does not allow administrators to log into their site. I've reported this but did not get a response.
If you have any thoughts please let me know. I'm anxious to get moving full speed with 3.0, and I'm glad to help with the testing efforts. I'm fairly capable with technical problems and troubleshooting, so it took me a while before coming to the point of asking for help!