Hyperspeed Slide Kicking
Mario is usually placed on the ground in the concave corner formed by 2 walls, so that he is running into the unreferenced one. By repeatedly slidekicking and recovering, Mario can build up a maximum of ~95 speed on normal floors and 282-400 speed[note 1] on slippery ones (at that speed he will break through the wall).
Shell Clone HSSK
Mario can also be placed on a shell clone instead of on the ground, as shell clones allow him to slidekick instantly without needing to crouch for a frame. Since he does not lose speed from crouching, the maximum speed achievable is again limited by the wall, so he can build up 282-400 speed[note 2].
- With axis aligned walls, the most distance Mario can move while still hitting both walls is when facing exactly 45° into the referenced wall, allowing 200*sqrt(2) ≈ 282.84 speed. With perfect wall projections with walls ~45° off from axis-aligned, it is possible for Mario to move parallel to the referenced wall while still touching its hitbox for 100 units, allowing 400 speed. [TODO images]
- check note 1, can't find any documentation for this xd