Shiffrin reeled off five consecutive World Cup victories to start 2018, but has since managed to only stand on the podium once in her last four outings, something compatriot Miller puts down to fatigue.
"She came in so fast and so strong from the pre-season, and all of her preparation was perfect," Miller told Reuters at his hotel in Pyeongchang, where he will work for broadcaster Eurosport as an analyst during the Games.
"And then on the road, racing every weekend and sometimes in the middle of the week, and training the whole time, sometimes you end up getting a little bit worn down," he explained.
Swiss oust great rivals Austria to win inaugural Alpine team event gold
Miller represented the US at five Olympics and became the oldest Alpine skier to medal when he won bronze in the Super G at the Sochi Games in 2014.
The outspoken 40-year-old said that Shiffrin might have been better off taking a few weeks off coming into the Olympics to give herself a chance to rest before carrying the considerable medal expectations of American fans.
"She's shouldering an enormous amount of pressure right now, regardless of whether she is racing or not -- she's that good," he added.
"Being one of the best in the world, you're going to take the majority of the pressure of all the World Cup, fans, media, everybody.
"You see it in every sport -- in basketball they're skipping games, or golf they're skipping events, Roger Federer in tennis. You have to manage yourself."
Miller's six medals, including gold in the super combined at the 2010 Vancouver Games, make him the most successful American Olympic skier in history, but the 22-year-old Shiffrin, who already has a gold of her own, could easily eclipse him.
What set her apart was not just her technique, but her physical power, according to Miller.
"I mean, the reality is that she doesn't beat girls by much at the top of the course... it's really at the bottom of the course that she tends to excel and move away from everyone else," he said.
"I think what separates her from everyone else is her fitness, she's able to stay at that high level all the way through to the finish, with no drop-off at all.
"I've seen it a lot -- she's good at the top, she's good in the middle and then she crushes everybody on the bottom."
Having officially retired last October, Miller is enjoying the switch from skier to broadcaster, as well as building his business interests outside of sport and taking care of his growing family.
His horse-breeding business is starting to mature, and with his Aztec clothing and Bomber skis, the one-time free spirit of the slopes seems to be becoming more comfortable in the boardroom.
"I do all the normal stuff, board meetings and budgeting, marketing and manufacturing, and distribution," he said.
"I have four kids and I try to play golf every once in a while. It's busy, but this is actually kind of a break, to come to the Olympics."
An avid hockey fan, Miller says he expects the Americans to top the medals table, led by his former Alpine skiing team mates.
"I think the women will have to carry us -- certainly, Lindsey (Vonn) and Mikaela are two powerhouses there. They're capable of winning five, six, seven medals between them," he said.