Back in 2010, Andrew Gardikis had clocked Super Mario Bros within five minutes, a feat he was given the opportunity to replicate in front of the game's creator, Shigeru Miyamoto. He wasn't able to pull it off sadly, but in the years since, he's refined his abilities and as of today, he's almost as fast as a machine.
In April, Gardikis posted the above clip which shows him beating the game in 4:58:09, which is pretty darn fast. What makes it even more impressive is that it's within 10 milliseconds of the world-record for the fastest tool-assisted speed run (TAS) of the game.
In order to hit the magical 4:57 time, Gardikis has to play the game perfectly. There's also something called the "21 frame rule", which results in a chunk of lost time (in terms of a speed run anyway) if you don't end a level at the right time:
Q12: What is the "21 frame rule?"
The game is always running a 21 frame long (0.35 seconds) cycle from the moment you power on the NES. In each stage, you can only gain or lose time in increments of 21 frames. At the end of each stage, the game determines which frame rule cycle you are in and rounds the time to the beginning of the 21 frame cycle. The only stage where this doesn't apply is 8-4 since the time ends at Bowser's axe before the rule can take place.
This is from a nifty FAQ attached to a previous run.