disclaimer: all times are measured on localhost, so you don't have net bottlenecks, and are the time taken from firebug, Firefox 16 on Ubuntu (not the "onload" time, that is the one taken to transfer the page and all the js and css, but the one taken to fully render the page) admin/default/design redirects to admin/default/site. that is variable with the number of applications, but in my case with 1 apps is 27kb (511ms, if no twitter box, 65ms). admin/default/design/someapp that is the main page of "edit" of a specific app again depends on how many files are in there, but for a medium-sized app is 100KB (welcome is 80.8kb, 126ms). Editing a controller, i.e. click on edit on default.py, loads a page of 14KB (1.06 sec).
If you hit ctrl+R at every page (that is, flush all the cached files): admin/default/site loads 225KB (1.48sec, just because there is the twitter box loaded, if not 569ms) admin/default/design/someapp loads 260KB (743ms) editing a controller loads 800kb (784ms) but usually this doesn't happen if not the first time you access the admin page. --

