Thank you for writing.
This is a known issue which is already logged in our Feedback Portal
. The issue is resolved and the fix is available in the latest version. Here are this item details: UI for Winforms Feedback Portal - IMPROVE. RadCheckedDropDownList - unchecking all items is slow
The issue page contains a workaround for older versions as well.
I hope this helps. Should you have any other questions do not hesitate to ask.
Check out the Windows Forms project converter, which aids the conversion process from standard Windows Forms applications written in C# or VB to Telerik UI for WinForms.For more information check out this blog post
and share your thoughts.