(idiomatic) Used to tell someone to stop doing something.
Used other than figuratively or idiomatically: see not, so, fast.
Source: en.wiktionary.orgI've run a lot of miles over the years, some fast and some not so fast. I've won some big races and I've had some big disappointments, but I enjoy the freedom of running and the challenge of training and competition as much now as when I first started back in high school. Alberto Salazar
A zealous soul without meekness is like a ship in a storm, in danger of wrecks. A meek soul without zeal, is like a ship in a calm, that moves not so fast as it ought. John M. Mason
For love she wist was weak without those arts, And slow; for jealousy is Cupid's food; For the swift steed runs not so fast alone, As when some strain, some strive him to outgone. Torquato Tasso