From power on through the end of World 3.
Two major highlights:
1. Glitch-hunter bonecrusher1022 found a way to gain height in a move I am calling a "hover glitch." When you exit the character select menu, your character gets positioned one pixel higher. Going back into the menu immediately afterwards preserves that pixel height gain and gives you another pixel of height. This can be spammed to gain infinite height and make shortcuts through various levels. Only one place proved to save any time: 1-2, precisely the very end of 1-2, enabling a 3 1/2 second time save. I cannot fathom another place where this saves time.
2. The world 3 boss is the biggest attraction in this game. Not only can you get a second hit on the boss when it re-enters the screen, cutting the time of the fight in half on its own, but there is also a death warp you can abuse if you touch the boss to die while it is moving left. The boss will be teleported to the left side of the stage and start the next step in its attack pattern, cutting another 5 seconds every time you do it from the right side of the stage.