Actually I'm not sure it is counter-intuitive. Think of a dual carriageway (and disregard for a moment the matter of junctions). If every vehicle is travelling at 50mph the capacity is maximised. If some are at 40mph, some at 50mph and some at 70mph with vehicles overtaking others the capacity reduces because moving from lane to lane both requires empty space (to move into) and further empty space (created by moving out). So on any given length of road there will be fewer vehicles.