Maemo 5 applications use GTK+/Hildon toolkit, some third party applications use plain Qt. This uses DirectUI(Harmattan UI framework), which is based on Qt's QGraphicsView. It's HW accelerated and designed specifically for resource constrained devices. GTK+ and plain Qt were designed for desktop applications in a time when such fluidity, animations or acceleration were not expected.

As I understand it, that's the outline. What exactly technically makes it so, I don't know.