slow horse, foolish fellow
駑 centers on the idea of a slow, inferior horse, and by extension a dull or foolish person. The two given meanings are linked by this shared sense of sluggishness and lack of quality.
駑 combines 馬 (horse) with 奴, which likely contributes the sound and carries a sense of low status or servitude, together suggesting an inferior horse.
A horse (馬) with a servant-like quality (奴) is a slow, low-grade horse. Picture a tired, sluggish horse that can barely keep up, and imagine calling a foolish person by the same name.
For ド, imagine a slow horse plodding along a dirt road: the 'do' sound of each hoofbeat (do, do, do) matches the reading ド, and the horse is too dull to go faster.
dull-witted; stupid; foolish
hack; inferior horse
to work hard; to keep one's nose to the grindstone; to whip a jaded horse
even an outstanding person becomes inferior when getting old