Another thing I checked is frame rate. Unfortunately, it is retrieved for hd-scrollable-group by clutter_get_default_frame_rate(). And the value is in theory set through clutter_set_default_frame_rate(), somewhere outside hildon-desktop because I couldn't find this function call in hildon-desktop.

However, the good news is we should be able to bypass this by hard coding the desired fps value to dir->manual_scroll_timeline variable. It looks to me this determines the number of frames drawn. But I don't know the what the graphic driver's refresh rate is so attempting to increasing the frame rate could:
1) Make the scrolling smoother (unlikely otherwise Nokia would have done it)
2) Make the scrolling slower because the request is too heavy on hardware.
3) Doesn't make any different due to graphic driver's limitation, either by refresh rate or something else (doesn't look like N900's refresh rate is slow, though.)

N900 doesn't do vsync, otherwise the frame_rate will be determined solely by vsync and in other words, refresh rate.

Maybe someone can try compiling hildon-desktop with the above modification and see if it works or not.