WordPress backend very slow since update

I run a wordpress-powered website (www.eclectro.nl), and have just updated from 2.3.3 tot 2.5. Since I’ve done so, the backend has become very slow.

Doing basic stuff in Firefox 3 beta 4 (such as going to the tab ‘manage’ or publishing a post) results in the browser loading, then doing nothing for 20 seconds, and then giving back the error saying that a javascript is taking too long to load (script: https://www.eclectro.nl/wp-includes/js/jquery/jquery.js?ver=1.2.3:11). In Internet Explorer 7, the browser says that the site is not responding for 20 seconds, and then still performs the requested action.

I’ve already deactived all my plugins, but that results in the same problem. I’m completely at a loss here. Any help is much appreciated!