Working with the Content Editor Web Part quite a bit lately I noticed a frequent (and annoying) error when committing changes to that web part. After making changes and clicking OK in the Web Part Properties Window IE would freeze and about 15-30 seconds later display the error below.
Are you sure you want to navigate away from the this page? The page took too long to save. You can click “Cancel”, and then try to save the page again. If you click “OK”, you might lose unsaved data. Press OK to Continue or Cancel to stay on the current page.
However, after you click OK to this warning your changes will be saved.
To solve this issue I added the site to the Local Intranet Zone in Internet Explorer’s Security Settings. Only add the sites that you trust since your username and password will automatically be passed on to the web server.