ok, this is not a real webmin or virtualmin issues at least for the performance issues, but on this board is so much brainpower, that I'll just give it a throw... ;)
We have a Athlon 64 3700+ running with 2GB RAM on 2 mirrored 300GB disks. Installed is debian etch AMD64, and the system is running XEN with a root domain of 256MB RAM hosting Postfix and Bind, the hosted domain is where the rest of the RAM is used and virtualmin is setup.
At the moment, there are 5 Domains, 3 of them using Joomla, one runnig a Board in addtion, ... all with very low traffic.
This just for a rough overview.
Ok, I notice some performance issues, and this not on the Joomla websites, webmin or the board, but on an installed Horde Webmailer and with phpmyadmin.
Just a few minutes ago, I tried to access usermin, had 5 times wrong passwords, but I am absolutely sure, that I entered at least 4 times the right one. Then I tried to access horde, veeeeeeeeeerrrrrrryyyyy slow response from the site, after 1,5 minutes or so I had the login screen.... logging in was about 3 Minutes to show up the inbox. In the meantime I tried to access phpmyadmin, also ... veeeeeeeeeerrrrrrryyyyy slow. The cpu useage was high, regarding to the system information in virtualmin.
I canceled horde access and then.. phpmyadmin was quick as expected.
So... I already looked up the mysql settings, raised table_cache a bit and so on, I don't see much of an issue there.
Now, after a while, Horde is fast... really fast. Uhm...
Strangely, I just tried another time to login to usermin... no success??? I am confused... it worked all the time, no changes besides translations done the last days....
Where would you suggest analyzing further on?