Not everyone likes the "life in the fast lane" approach. They may keep themselves calm and their blood pressure low by taking it easy, arriving early so there's no anxiety the plane will be missed, walking slowly to avoid getting all sweaty (or perhaps because of a health problem?), etc. The checking of a bag may be to avoid the anxiety of not having overhead compartment space (especially if not in F), or because they pack a lot of stuff they need or think they need.

Of course, if this slowness results in a missed connection, or a missed meeting, or something else serious, you (or his boss) may need to have a friendly word with him about future trip planning (or perhaps looking for a job that doesn't require allegro behavior with a largo personality )