I have a Tabstrip with 2 tabs.

In each tab I have 10 radtextbox (multiline, with message for empty message). The first tab doesnt have any problems for moving among the textboxes, but when I select the second tab, I experiment a 2-3 seconds for moving among the textboxes.

Any suggestion?

You could look into using a RadInputManager on your page. I'm not sure what you are looking for from a RadTextBox but you could use a RadInputManager with regular asp.net textboxes and get a lot of functionality with high performance. You can read over this documentation article which talks a bit more about the performance gains from using RadInputManager.