I can't remember how long it took me to finish Half-Life 2. It's been a while, after all. But I can say, with absolute confidence, that it was a lot longer than the breathtaking 40:49 it took the SourceRuns Team to do it.
As is usual with speedruns, this is not a typical playthrough of Half-Life 2. It's done using a 2006 build of the game that used the original engine, which has “significant movement differences, as well as a long list tricks and glitches” that have since been patched out. It's also “segmented,” which basically means that it's a group effort: Different players hammer through different bits of the game, and the best of them are stitched together into what you see in the video.
The net result is fast, furious, and funky, as the runners clip through walls, fly over levels, and blow past the talkie bits. It doesn't look like much fun in the conventional videogame sense, but it's a hell of a sightseeing tour, and a remarkable accomplishment, coming in at just one-third of the world-record mark the SourceRuns team set in 2013.
A spreadsheet that breaks down just about every element of the run you can imagine is up on Google Docs, and the SourceRunners have also posted a separate video explaining how clipping works on YouTube.