"At some point a sinner will be punished; many decisions or events that are important in one's life take time in coming. Some 1,600 years ago the Greek philosopher Sextus Empiricus wrote: 'The mills of the gods grind slowly, but they grind small.'
One of Longfellow's translations was a 17th century poem, 'Retribution,' by Friedrich Von Logau:
'Though the mills of God grind slowly, yet they grind exceeding small;
Though with patience he stands waiting, with exactness grinds he all.'
From "The Dictionary of Cliches" by James Rogers (Wings Books, Originally New York: Facts on File Publications, 1985); quoted in https://www.phrases.org.uk/bulletin_board/59/messages/502.html