This is what I did:
1. Create an element with no template (that may not be important)
2. Create a second element (also happens to be without a template)
3. Add an extended property to the second template
4. Configure that extended property to be a reference to another element and choose the first element you created
When you try to close the dialog, it will not close and will show "Extended Properties (Not Responding)" in the dialog's banner
From this point on, things are very slow. Eventually (5 minutes) you will see a dialog saying "The request operation ... did not receive a reply within the configured timeout (00:05:00)"
If you try to uncheckout the change you made (adding the elements and the extended property), it will take a long time to respond as well and eventually display the dialog as shown above. The second or third time you try to uncheckout, it will eventually succeed and performance will go back to normal