From the "Performance Tips" section of the manual:
Changing Text objects every tick
Change the size or text of a Text object every tick - even just for an animation or transition - will likely produce poor performance, especially on mobile devices. The problem is even worse if the text object is large. Text rendering is very fast as long as the object is not changing, but upon changing the object must do a relatively expensive redraw of the text and replacement of the cached texture. Try to use small Text objects that do not change regularly. If you must change text regularly, consider using a Sprite Font instead, which is fast even when it changes every tick.