VDL2 generally gives much lower latency than SATCOM or HFDL, which is why ATN CPDLC in Europe often feels almost instantaneous.
But I suspect the big latency differences are less about raw transmission speed and more about the underlying network architecture: ATN/ACARS routing, DSP congestion, store-and-forward logic, and possible transitions between VDL2 and ACARS paths.
Crossing VHF coverage boundaries and switching between VDL2 ground stations could probably add short delays too, although that alone may not explain the occasional 20–40 second delivery times.