There are some Transitions that are pretty CPU demanding using shaders and such so they could cripple the CPU while playing on large areas. However they would not freeze at the beginning and continue afterwards so there is high chance the problem to be in the RitchTextBox.
Please if you find the problem is in the transition control get back to us.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!