Some swimmers analyzing the available data say Nyad, who has said she tends to swim at a speed of roughly 1.5 mph, appeared to maintain sprinter's pace or faster for a considerable amount of time.
Navigator John Bartlett said the increased speed was due to the Gulf Stream working in her favor, nothing more.
"At some points we were doing almost 4 miles an hour," Bartlett said. "That's just the way it works. If the current is in your favor at all, that explains it."
Some of Nyad's critics also question whether she violated the traditions of her sport - many follow strict guidelines known as the English Channel rules - by using a specialized mask and bodysuit to protect herself from jellyfish.
Nyad never said she would follow English Channel rules, and she wore a full, non-neoprene bodysuit, gloves, booties and a silicone mask at night, when jellyfish are a particular problem, and removed the suit once she got over the reef on her approach to Key West.
The data collected by Bartlett and two observers will be submitted to three open-water swimming associations and the Guinness World Records for verification, Bartlett said.