Thank you for writing.
Indeed, loading multiple themes initially may take some time. This is why I would suggest to load the themes in a separate thread, once the application is loaded. This way, only the default theme will load upon start up, and the rest of the themes will load afterwards, not affecting the initial load time. Please refer to the following KB article, demonstrating how to take advantage of this scenario: http://www.telerik.com/support/kb/winforms/themes/load-theme-components-in-a-separate-thread.aspx
I hope that you find this information useful. Should you have any other questions, do not hesitate to contact us.
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.