Thank you for contacting us.
I tested several scenarios, but I was not able to reproduce any performance issues regarding RadComboBox. Please make sure that you have followed the optimization suggestions mentioned in these Knowledge Base articles: Optimizing RadComboBox and RadListBox performance
, Using UI Virtualization in RadComboBox and RadListBox
As to the Output, the messages are normal since you run your project in Debug mode, but the assemblies referenced are in Release mode.
If you continue experiencing the issue, please open a new support ticket and send me a sample project which reproduces it. This will allow me to assist you further.
the Telerik team
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