Note: These secrets where found during the competition and none of them by myself.

Cloud in 1-8 (found by Stacked_Like_Pancakes)

Get behind white block on 1-8 and go back to the left. Go between the pipe and munchers and run behind the Thwomp. Press UP to get into the door Use the cloud to skip 1-9

Hammer Suit in 1-9 In the last section of 1-9, go all the way left and hit a hidden block. Jump on it and find other hidden blocks until you make your way to secret platform.

P-Wing in 1-5 (found by Zmoxer) Collect each of the 30 coins in 1-5 and a white mushroom house appears. Use the P-Wing in the autoscroller to fly up the coin trail and into a pipe that spits you out at the end of the level

Other Speed strats

You can reliably do a wall jump in Fort 2 which saves you climbing up the stair plattform pattern. See the WR video for that.

After beating bowser, you can also finish the game by standing on the lower floor just right below the actual exit door. You can still enter the door as soon as it is visible.