That said, it is also a great piece of advice for the multilingual semiliterate. Consider:

A Slovak woman, fluent in Slovak, and reasonably adept in English, wished to communicate with her granddaughter in America, who is fluent in English, and has some proficiency in Slovak. Each one has the same choice; to write in her native tongue, and give the recipient the headache of translation, or to do the translation first.

Since the communication will be more rich if each writes in her native tongue, they do so. But if they write fast, they'll tend to take shortcuts in penmanship that will make translation difficult for the recipient. So they "write slow," to ensure that they clearly spell each word.