When loading a page longer than the height of the scrolled window, the entire page is rendered without scrollbars, and then has to be re-flowed and re-rendered once scrollbars are made visible, in order to fit the new allocation. This slows things down, particularly on mobile devices. Perhaps this can be easily fixed by twiddling the order in which re-layout, rendering and scrollbar updates are done?
This was probably fixed when Holger reworked frames and ScrollView. Can't reproduce the issue any more.