periwinkle | [TAS] SMB1 Minus World (non-FDS) frame save + setup in description (1:35.559 RTA timing) @periwinkle. | Uploaded July 2021 | Updated October 2024, 5 hours ago.
So much for "1:35.576 is the perfect Minus World speedrun"...
By adjusting Mario's subpixel positioning to take advantage of the whirlpools, I managed to save one frame over the "perfect" speedrun. Here is a "human theory" TAS demonstrating this frame save.

Setup: YOU MUST GET R14 EXITING 1-2. This is important because whirlpools affect you based on the parity of the frame counter, which starts counting from the start of the black screen after you enter the warp pipe in 1-2.
(You'll get 1:35.593 if you do this setup with R13.)

Upon entering -1, hold D+R and then swim on the first frame that you touch the ground. (In other words, you should touch the ground for exactly one frame.) Then play the rest of the level normally without slowing down. The exit pipe has about 6 subpixels of leeway so as long as you don't get a terrible exit, it shouldn't be too difficult to get this frame.
Alternatively, instead of doing a frame-perfect input like that you could just do a wiggle that loses between 5-11 subpixels, anywhere before the first whirlpool. Note that the more you wiggle, the less leeway you will have on the final pipe entry.

(Fun fact: By further manipulating your position in the final whirlpool, it is possible to get 1 subpixel away from another frame save. I don't think that frame is possible, though.)