There is possibly also a very non-obvious reason why your admin interface is very slow. Magento has a module named Mage_AdminNotification. Try to disable that ext. Because what it does is query magentocommerce.com for new update messages. If their servers are slow your admin page waits and is in effect slow because of the network lag and loading of the external news. If you have secured your outgoing server connection through a firewall this can be even more frustrating, since the admin interface will wait for the timeout when it cannot reach magentocommerce.com
To disable it: go to System -> Configuration, scroll to the bottom and hit Advanced(in the Advanced section). Now disable Mage_AdminNotification and save!
