Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"Do not play this piece fast. It is never right to play Ragtime fast..." Scott Joplin
What's up with the epigraph?
The epigraph speaks to the steady, slow pace of life at the turn of the century. Ragtime takes place befoe World War I, before technology and industry and a changing society created an America that moved at a much faster pace.
But, although America between 1900-1917 isn't moving to the frenetic pace of hip-hop, or house, or even 1950s rock n' roll, it is moving to the peppy strains of ragtime. Gone are the days of the waltzes: things are speeding up.
Ragtime music was a precursor to jazz, which was a precursor to rock n' roll, which was a precursor to… well, to most everything else. Although this epigraph suggests playing ragtime music slowly, Doctorow knows that ragtime will pave the way for the hedonism of the Jazz Age and everything that came afterwards. Change is inevitable, however slowly you want your music to be played.