According to Germany's Auto Motor und Sport, the competition has concluded that Red Bull's advantage is not due to having a superior crew, or having simply invested more time practicing.
"One or two fantastic stops, you can imagine that," McLaren sporting director Sam Michael is quoted as saying.
"But five, with that consistency -- that's really impressive."
The suspicion is that Red Bull has stepped up a gear with its pitstop equipment and technology.
"Our guys are certainly not inferior to Red Bull's, and all of our training methods have been optimised," said Michael.
Analysis shows that Red Bull has made a half-second per pistop time improvement in 2013 compared to last season.
"You can make a jump like that with developments in the equipment," Michael said.
One theory is that Red Bull is using higher air pressure for the wheel-nut guns.
Indeed, every time a Red Bull crew member uses his gun, he dutifully puts it away in a bag, suggesting they are hiding a secret.
"You can hear that they are using more pressure than the others," Williams team manager Dickie Stanford said.
But Michael disagrees: "I doubt anyone is going over 30 bar. Maybe they've simplified the switching mechanism, and it's now electronic rather than mechanical.
"Things like that could save you time."
Stanford, meanwhile, admitted Williams' investigation shows that the Red Bull crew is so easily able to throw the new wheels onto the car.
"At Red Bull they're throwing it on and apparently not even knowing if it's properly seated. It (the wheel) seems to go on by itself," he is quoted as saying.
That would imply that Red Bull has perfected the design of the hub, wheels and nuts so that the wheel is always perfectly seated when the wheel gun is pressed on.