SMB can be completed without ever pressing Left. This TAS demonstrates the fastest way possible. It's interesting that the fastest route happens to show entering 5-1 through 1-2 wrong warp glitch.
Authors (who have their Left button broken): HappyLee, Kriller37, Mars608 & KFCMario
HappyLee: I started this project back in 2009. Mars608 and KFCMario joined the project mostly during 2012 to 2015. We've made several demo runs, but the project was postponed many times simply because a Firebar in 6-4 always gets in our way. I felt that there should be a way to avoid that.
I restarted this project in 2020 with Kriller37. The 6-4 Firebar problem was solved with 5-1 Bullet Bill glitch ending, which is the hardest and the last piece of the puzzle. Killer37 finished many stages like 5-3, 7-1, 8-1 and 8-2, and his great skills and high efficiency make this TAS finally see the light of the day in 2020.
Q: Why do you slow down in many places?
A: Most of the slow downs are for entertainment, and some to avoid certain timer. This game has 21 frame rule, so slowing down doesn't necessarily mean time loss.
Q: Can this be done by real-time speedrunners?
A: I believe YES, because SMB speedrunners nowadays have done crazier things. There's no super-hard glitch involved in this route. I'd certainly love to see a "no Left" real-time speedrun someday. :)