He could be riding an elephant and it wouldn't matter. The bare legal minimum is not the standard for what constitutes good behavior He'd being dis-courteous to other road users. He shouldn't complain when some of them let him know.

Regardless of his mode of transit if he's going below the prevailing speed and avoiding taking opportunities to let others pass he deserves to hear about it in a not nice way.

Not that it's applicable here but it's doubly true so if he could simply go faster and conform to traffic norms but chooses not to (this is why a speed limit following Prius catches more shit than a laden truck that can only go that fast).