I was working recently on a local server without internet connection (nothing can stop us !! ;)) and i noticed that Magento administration is extremely slow. Some parts of administration took up to 10 seconds to load.
On every page view in admin Magento was trying to resolve widgets.magentocommerce.com address and connect to it few times which caused time delay. His notification system was trying to grab new message.
The solution was to disable Mage_AdminNotification module and since i was offline i didn’t really need it. You can do it in
System -> Advanced -> Disable modules output
Some other admin parts which are using online resources will still be slower, like Dashboard for example because it is using google charts so it’s trying to connect to that, but everything else will work fine.