“Are you sure you want to navigate away from 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.”
Notice that IE7 works properly – i.e. there are no any warnings and it works fast enough. So 1st I tried to launch the page in Compatibility View by setting “Display all websites in Compatibility View” in Tools > Compatibility View Settings:
But it didn’t help – mentioned issue didn’t disappear. Also I tried to change Browser Mode and Document Mode in Developer Tools – and it didn’t work also:
During further investigation I found the following blog post: http://www.chakkaradeep.com/post/2009/07/11/IE8-to-SharePoint-e28098Are-you-sure-you-want-to-navigate-away-from-this-pagee28099.aspx. It describes solution which worked for us: in order to fix it you should go to Tools > Internet Options > Advanced and turn off "Enable native XMLHTTP support" option:
After this warning disappeared and IE8 started to work as fast as IE7 in edit mode.
The remaining question for me: is why Compatibility View didn’t help in this situation? Seems like Compatibility View doesn’t give compatibility for all features, e.g. native XMLHTTP support still works differently in IE8 even in Compatibility View.