Fast acceleration is not that hard, you jump left and then right, and it's quicker than just running to the right. The issue is that fast acceleration on that level only gives you enough time to reach the next framerule if you do the first fast acceleration jump on the first frame of the level appearing. Someone has done it with a savestate just on that level, but doing it in an actual run will be even harder.
Yeah that's the clip I mean. There was a setup found for clipping into the top of the pipe rather than the top of the pipe entrance that does not lose time over the entrance clip. It is significant because the entrance clip is hard as shit.
Minus world on NES only has one level, but on the Famicom there are three minus levels, and the last one has Bowser's axe at the end. Touching it will bring up a princess is in another castle message, but it is considered to be the end of the game because you can't progress past that point. There is also an underwater flagpole glitch in minus world that skips the entire walking to the castle animation, which is pretty cool.
I think the only real time save left in Mario is a fast acceleration in 4-1 that has only been achieved by a human once, flagpole glitch in 8-1 and whatever time there is to be scraped from 8-4. 8-4 does not work on framerules since you don't need to wait for a level to load after it, so there is always time to save in that level, but I believe the current record has the fastest 8-4 there has been in a record run.