Is colon cancer fast-growing?

Can be. The fastest-growers are the ones that follow genetic pathways associated with the lynch loci. They may not even be preceded by a polyp -- colonoscopy can be clean one year, and a year later, there's a cancer. That's actually quite fast as cancers go. Hope this helps.
Depends. Most but not all colon cancers start as a small polyp which gradually over many years may grow developing progressive genetic changes eventually developing dysplasia followed by cancer. Once it is a colon cancer doubling time (doubling in size ) typically varies from between 30 to 300 days.