Why does ゆっくり take the honorific prefix ご? Is the only mimetic expression that accepts the honorific prefix ? And for that matter, why does it take the honorific prefix - which is usually meant for ?

There are a number of exceptions regarding the usage of /, and, ultimately, you have to learn them individually.

According to a survey by Tanaka (1972) based on the word usage of newspapers, attaches to a kango roughly 20% of the time, but very rarely attaches to a wago (they identified only and ). Even a long academic article dedicated for this topic lists only , and . is mimetic, so is **not** the only mimetic expression that accepts . Still, you can say the number of such words is very small.

Kango that take tend to be common and daily words such as , , , and . A few words accept both and , and in such cases sounds stiffer (e.g. vs , vs ). FWIW, I feel , and also have formal, polite or even a little pompous overtones.