I've created a Silverlight project with huge xaml but the designer stayed responsive. I believe your case is a bit more specific and I'm shooting in the dark while trying to simulate it.
Could you try to modify your solution in order to provide us a sample without the confidential stuff?
The issue you describe should be related to the Silverlight projects and more specifically with the currently opened xaml file you are editing. I think the issue should be still reproducible if you isolate the Silverlight part of the solution in another solution without the confidential data and without the 12 .Net projects.
In order to validate my suggestion you could unload projects except the one with currently opened xaml file and check if the editor is responsive or not. If my thesis is right you should observe performance degradation with unloaded .Net projects.
On a side note, I just wanted to encourage you to take advantage of the support ticketing system (http://www.telerik.com/account/support-tickets/new-support-ticket.aspx) in cases when you need a prompt response to urgent issues. This is the best way to reach our support staff and attach sample solutions.
the Telerik team
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