Dayna left the Free Zone on her Vespa. On a bike she could weave in & out of any traffic jams she encountered. Her route was shorter as she went via Loveland Pass (she was seen off from Sunset Amphitheater). It's a clear shot from Boulder to Las Vegas via a slice of southern Utah that way, less than 800 mi. She was also only in her 20s with no physical ailments, thus more energy & stamina than a man in his 70s. She could easily have arrived in Vegas in 2-3 days.
Judge Ferris, on the other hand, left in a big-ass truck with a chainfall attachment, so that he could push & pull his way out of traffic jams. That meant delays for him. So did the muddy road conditions. The Judge suffered from arthritis, which was worsened by constantly getting in & out of the cab to attach the chainfall to yank vehicles out of his way & the cold, rainy weather he encountered on his route. The night he saw the crow, he was delighted to have found a motel with a generator so that he could crank up the electric heat in his room & warm up his stiffened hands.
His route was vastly different. He drove thru Colorado & Wyoming to Idaho. Probably close to (if not more than) triple the mileage of Dayna's route had he made it to Vegas. The Judge himself decided that route. He wanted to see what he could see in more of Flagg's territory.
(Tom Cullen went on a regular old bicycle, & on the same route as Dayna. He was somewhere in the neighborhood of 40, & it was mentioned several times that he was a physically impressive specimen with the stamina of a bull. He also would've beat the Judge to Vegas.)