First, the obvious. Miniland is very, very good at Super Mario Bros., and plays through pretty much the entire game without ever coming to a stop. But even getting Mario to and past Bowser without an enemy so much as ruffling a single mustache hair just wasn't fast enough.
Obviously, the player is going to utilize Mario's Warp Zones. Now, that's obviously a massive time save, but speedrunners decided that GETTING to this zone that let you skip almost half the game was simply too slow. So they figured out a way to clip straight through the wall into where the room was hidden.
If you're thinking this looks convenient and want to dust off the NES and give it a try, go ahead! All you'll have to do is go through the entire level without ever slowing down, and then when you get to the end, jump on the exact right frame to clip into this pipe. Given that Super Mario Bros. runs at 60 frames per second, you have all of 1/60th of a second to do that. If you don't get it, no problem, just restart the entire game.
That should give you some idea of the wiggle room you have to work with when attempting a world record speedrun of Mario Brothers, which is about the wiggle room afforded to a mosquito trapped in a block of amber.