A heavily encumbered character may be greatly reduced in the number of AP they possess in a given round, reducing their speed to less than one foot per second. This is also meant to emphasize the difficulty in acting under a heavy load and being situationally aware of all that is going on around the combatant.
Obviously, Caleb is not required to take this route - though there may be a specific reason he needed to observe things that could only be spied from 0204 or 0302, or to avoid setting off a trap by carefully following a route that depresses certain floor stones underfoot (which also would not cost AP).
Faster movement may be accomplished through running. In order to run the next round, the combatant must expend at least 50% of the AP in moving forward (in one direction), regardless of the combatant's actual speed. Thus, if the combatant only has 2 AP total, and spends 1 of them moving forward, then that combatant will be able to break into a run the next round. On the other hand, if a combatant had 5 AP and only spent 2 AP moving forward, then running the next round would not be an option.
Drawing a Weapon While Moving
Movement in Combat