This is not an unusual problem for new implementations of text boxes. It has to do with the algorithm used for calculating line breaks. Based on the kind of behaviour you're reporting, it sounds like the efficiency of the algorithm they use is strongly dependent on the length or words (i.e. it might be, say, O(n^2) with respect to the length of words). You should report this directly to Microsoft (with a clear example) so that they can fix the bug.
Chris Fraley
2010-09-22 23:47:08