Contact rates as high as 500 per hour can be achieved using the exceptional spectral and temporal control offered by digital radio. pdf 
I was impressed to observe one station completing contacts at the rate of one per minute. Time limited contests favor such efficiency. But the DXpedition, traveling to a remote location to put a new "country" on the air, demands even more efficiency.
In WFJT-X FT8 DXpedition mode contacts between the remote “Fox” and calling stations, the “Hounds”, can be completed with as little as one Fox transmission. Moreover, authorized Foxes can transmit up to five signals simultaneously, thereby allowing contact rates up to about 500 per hour in ideal conditions.
WSJT-X maintains several queues in a manner that allows difficult QSOs to be completed while keeping the overall QSO rate high. We use a “3 strikes and you’re out” rule. Fox will call a specific Hound up to 3 times, waiting for an “R+rpt” response. If a Hound repeatedly sends an “R+rpt” message, Fox will send RR73 up to 3 times. Finally, the total timespan of an attempted QSO is limited to 3 minutes. When any of these timeouts is exceeded, the QSO is aborted.