Why coastal convoys had no simple average speed

What was the average speed of a British coastal convoy?

A convoy generally moved at the speed of its slowest ship, while age, loading, maintenance, and weather made a single reliable average misleading.

Published maximum speeds assume machinery in good condition and a ship able to sustain that performance. Coastal formations mixed many vessel types and states of repair, so the slowest practical performer set the pace. This compounded the escort problem by increasing exposure to aircraft, submarines, and fast attack craft.