Even though Shiffrin eased off slightly on the second run, when her time was bettered by two other skiers, nobody else could get near her total time of one minute 43.50.
Shiffrin, the overall World Cup champion and Sochi 2014 Olympic slalom gold medallist, holds a huge lead with her tally of 680.
Shiffrin, who was suffering from a cold, lost more than half a second of a big first-run lead but still beat world GS champion Tessa Worley of France by 0.31 seconds. She was 1.64 seconds ahead of runner-up Petra Vlhova of Slovakia, who won the last two World Cup slaloms.
"I let it go down the hill and I am really, really happy with my skiing that run", said Shiffrin, who also triumphed in Saturday's giant slalom on the same course.
He now has a 99-point lead over Kristoffersen in the slalom, while his overall advantage stands at 154 from the same rival.
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"My first run was really really good and my second run was really good too", Shiffrin told reporters. That number leaves her one short of the all-time record for most World Cup wins by a 22-year-old, set by Austrian great Annemarie Moser-Proell in the 1970s. "The surface is incredible and froze overnight so it's just so much fun to ski".
While a night race in Flachau on Tuesday will be the penultimate slalom ahead of the Olympics, Shiffrin will be eager to keep her momentum going in weeks to come. "I feel so like in a good place, it doesn't feel like I am dreaming, it doesn't feel like it's something insane that's happening".
The Eagle Vail, Colorado, native has been on a run for the ages this season, proving dominant in just about every event and is the world cup leader overall, in slalom, giant slalom and downhill.
"I felt so clean and aggressive on the first run", Shiffrin said.
Shiffrin probably won't win that handful of gold medals, but mainly because she is unlikely to enter five different events in South Korea.