I confirm that the visual glitch and performance degradation depends on the number of RadTextBox controls available in the page. This problem is not observed with RadTextBoxControl or MS TextBox.
Currently, the possible solution that I can suggest is to use a RadTextBoxControl instead of a RadTextBox.
I hope this information helps.
Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik
Get quickly onboarded and successful with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers.Learn More.