This is something that has happened since the last time I touched a site a few months ago. The menus page on the dashboard will only load the first column, which makes it useless. It churns for a long time, then finally seems to time out. Firebug shows this script error:
ReferenceError: columns is not defined
It happens in Firefox, IE, and Chrome, exactly the same way. I've disabled all the plugins and cleared the cache. Nothing seems to help. I'm on 3.5.1.
Nothing looks amiss on the site, only on the dashboard.