I assume you're updating the textbox from your background thread, and what you're seeing is that the textbox is not updating until your main thread finishes its time critical task.
This is probably happening because of the way in which you're drawing to the text box. You might want to read through this
thread for a discussion of how different types of UI actions affect threads that are not processing events. The bottom line is that if you use the SetCtrlVal to update the textbox, you should be able to see the textbox update even while its thread is blocked. See the attached program for an example. In this same attached program, if you replace SetCtrlVal with InsertTextBoxLine, for example, you'll notice that the textbox will not update until its thread is free to process events.
Hope this helps.
Luis