There are known cases of people getting super fast PRs - I believe the record is under 6 days, possibly 2-3 if I remember correctly.
However, there are many factors which could affect this.
Often these are cases with prior refusals due to Cap in 2014 or due to some missing doc. This means other things for such candidates were already processed.
Also other factors like a remote embassy being relatively free, single young applicant, with little travel and proper education / experience, uncommon name to get through background / security check easily, belonging to a country with less security scrutiny etc. - there are so many elements that even CIC itself cannot predict processing times for every one. Those are just approximates for the average case.
