Why would a perfectly healthy person have a colon with slow transit?

Many reasons. Undiagnosed hypothyroidism, slow gut motility (squeezing) in general, or illnesses elsewhere in the GI tract can cause slow motility. Get your gallbladder checked for stones, as this can rarely be a cause of slow motility. This disorder is best treated with bulking laxatives (benefiber), an osmotic laxative (miralax), and sometimes with colon-stimulating drugs. Good luck!