Scabs are actually very slow to form. First your blood needs to clot, which forms the base for a scab to form. Clots are like the first floor to a skyscraper, without it, you cannot have a skyscraper. Scabs would be the top floor of a building, they are built last and make the skyscraper complete. A scab is really just a dry clot but you need a ton of clots to make a scab (like you need a ton of floors to make a skyscraper). Clotting happens in a few minutes whereas a scab would take maybe about 10 minutes, once the clot has dried out.