Brexit is the cause.Blaming Brexit, still, is pointless trolling. You may as well hark back to times before international terrorism as times before Brexit.
There's a known capacity on travel by that route. The boat operators know what the customs capacity is, and should be allowing a factor for typical French customs cooperation or lack of it.
Capacity partly depends on the number of boats, or more wouldn't have been put on at the last minute to ease the situation.
This is just British mismanagement.
every passport now has to be checked, scanned, stamped.
Dover infrastructure was built for flow rates designed for EU membership.
if the UK rejoined the single market, the problem would disappear.