we have found some performance problems when we add a component that is modal on the Viewport. This seems something related to this post http://www.sencha.com/forum/showthre...-on-the-screen .
This problem arises also every time a selectfield is pressed and the relative picker is displayed.
I've attached to this post a test page that reproduce this issue (i've renamed .txt the file, but is an html file).
Using the SpeedTracer chrome plugin to profile the browser when pressing the selectfield, reveals that almost all the time is spent recalculating layout positioning of elements and not in js code.