VGA devices are slower because it requires 4x the processing power form the video chip and 4x video memory compared to standard QVGA devices. So it's mostly a hardware issue and only somewhat a software issue.
Unless you're gaming, the VGA machines are generally decent performers, though the X50v is the slowest at graphics among VGA devices, with the iPAQ hx4705 being the best and the ASUS in the middle. The Dell is a poor performer even though it has impressive graphics hardware because that chip was designed to work with a kind of graphics library not commonly used on Pocket PCs at this time. So in the future things might improve for the Dell if folks decide to write optimized versions of their apps and if Dell improves their driver a bit.