I'm not sure if donkeys even affect the movement-speed on the map?
They give you the same speed bonus as rouncey on the map, only sumpters are slow as far as I know. I should be moving 39m with the amount of troops I have and no goods or 33 with goods.
When I first noticed the issue I was leaving a fief with less goods than horses/troops and was moving 24, dropped the goods off in the fief then started moving 26m so something's up. Zero crates by the way.