I think you are definitely on to something. An increase of 1 mile an hour is not a straight percentage in effort. I can cruise indefinitely at 16 or so MPH - (like 80 kilometers) but push it out to 18 and I rather doubt the same is true (I ride a MTB - but lighter than yours).

Just making your commute is enough of an accomplishment. You might in a future find your speed creeping back up as your body recovers too. I know that if I am dragging physically, it is nearly painful to keep up a high rate of speed. If I am rested and feeling great, 10% more is common. I just let my body dictate. I am not in a race with anyone that I know of.