As of November of last year the world record time for completing Halo: Combat Evolved on Legendary difficulty was 1:29:59. In this video a group of four speedrunners demonstrate techniques that shave a full 22 minutes off of that time.
A group effort by a group of Halo speedsters which includes former Legendary record holder Andrew "goatrope" Halabourda, the record-mocking 1:07:04 run above is the end result of a project that's been in the works for the better part of a year now.
What took so long? For one, this is a segmented run, which means the runners were able to attempt the portions they were responsible for again and again until they achieved the best possible time. Once they were finished, the run was stitched together to represent a complete play through of the game. The runners represented in the video are familiar to anyone following the Halo speedrunning scene—Scurty, Whistle, Vetro and Sol—four of the fastest players on the planet.
That also means the chances of this run being replicated by one player in a single play through of the game are next to nothing. Barring any massive undiscovered glitches, this might be as fast as Halo on Legendary can get.