Unfortunately DXB has not progressed as far as automated loading in terms of robots for bags to bins as of yet. Actually this is quite an expensive undertaking and considering the labor costs here does not make sense. Not only that but the dynamics of baggage that pass through in terms of shape, does not really allow for this anyway as we encounter all weird shapes, especially through Asia.
As for the bags being directed, they will head to the specified carousel where the ULD's are lined up alongside thereof. The bags have to have their associated tags scanned and linked to the specific ULD (which is how the guys know where to find it when the pax goes walkabouts).
In terms of speed, it doesn't really matter whether they go into a ULD or mail trolley. If anything, a mail trolley is slightly quicker to load as you don't have to worry about fitting the piece within the unit itself as they will just be removed at the aircraft side anyway.