Why coastal shipping losses were so high

Were coastal shipping losses mainly caused by the slow speed of the ships?

Not by speed alone; coastal convoys remained exposed to submarines, mines, Luftwaffe aircraft, and E-boats throughout the campaign.

Slow ships increased exposure, but they do not by themselves explain the casualty rate. Coastal waters brought several German attack methods into the same confined routes: smaller U-boats could hunt or lay mines, aircraft could strike from nearby bases, and Schnellboote could attack at speed. The answer shifts attention from ship design to the unusually crowded threat environment around Britain.