I haven't played The Witness yet, but nearly everyone else at PC Gamer has. It's notoriously tricky, so it's a surprise to see impressive speed runs trickling through already. The current record is held by speedrunner Azorae, who has managed to complete the game in 25 minutes and 53 seconds. Contrast that with the work of fellow PC Gamer scribe Chris Livingston, who (in his own words) "spent 23 minutes driving a boat back and forth past a rock I thought had something to do with something, but didn't."
In fairness to Chris, Azorae has clearly memorized the solution to all of The Witness's puzzles – which is quite a feat as I can't even remember how to spell "occasionally". Check out the video below, and be warned that it contains spoilers for pretty much the whole game.