The Dockers broke the game open with a nine-goal blitz in the second quarter and powered away to a 24.11 (155) to 4.7 (31) victory at Bunbury's Hands Oval on Saturday.
The even spread of contributors particularly pleased Longmuir, with Andrew Brayshaw (30 disposals), Shai Bolton (27), Murphy Reid (23), Luke Jackson (19) and Neil Erasmus (22) all influential.
Josh Treacy (four goals), Patrick Voss (four), Sam Switkowski (three) and Jackson (three) led the scoring as the Dockers kicked the last 19 goals of the game and improved their percentage from 135.2 to 147.9.
Star midfielder Hayden Young finished with 23 possessions and two goals in a strong return after missing three weeks with concussion.
The dominant performance ensured Fremantle (12-1) will spend a third consecutive week on top of the ladder in pursuit of their maiden premiership.
The Dockers have the bye next round, after which Caleb Serong, Brennan Cox and Matthew Johnson are all set to return from injuries to bolster the competition's hottest team.
"We can get better in personnel, but there's still improvement (in the group). We're not our best version yet," Longmuir said.
"The challenge for us as a playing group is to remain hungry and keep searching for areas we can improve, because everyone below us on the ladder is doing exactly that.
"So I don't sit here and think we're our best version yet.
"Through personnel and the way we're playing, we've got areas of our game that we can get better at, and that's what we've got to strive to do."
North Melbourne (5-7), meanwhile, slipped further back in the race for a wildcard spot and lost defender Riley Hardeman to concussion before halftime.
Harry Sheezel (27 disposals), Luke Parker (26) and Luke Davies-Uniacke (20) fought hard in a losing side.
After beating West Coast in their inaugural Bunbury "home'' fixture last year - under a deal with the WA government - the Kangaroos stayed with Fremantle for about 40 minutes.
The margin was just six points when George Wardlaw kicked North's fourth major at the seven-minute mark of the second term.
But the Dockers went on a tear, slamming through the next eight goals in just 19 minutes to lead by 54 points - 13.4 to 4.4 - at the main break.
It was Fremantle's highest first-half score in more than 11 years and also the third consecutive game in which the Kangaroos have conceded 75 or more points by halftime.
There was no repeat of North's remarkable fightback against Gold Coast last time out, as they went goalless in the second half against the Dockers and added just 0.3 to their tally.
It was the equal-seventh heaviest loss in Kangaroos history and the biggest margin of defeat in Alastair Clarkson's coaching tenure.
"Fremantle were good and we were dreadful," Clarkson said.
"Perhaps you have a little bit of shame because we want to come over here and be putting on a better performance than we did today.
"That sat us on the back foot early in terms of contest and we never really recovered from it."
Clarkson conceded the Kangaroos failed to adequately handle the break in routine during their mid-season bye last round.
"How often do we see that, that a team is just so poor coming back from a bye and having a rest?" he said.
"You get out of your routine and you get out of your resilient, tough approach.
"We were just nowhere in that space today."
