When I watch Mario speedruns, I noticed that you could only do a fast acceleration on level 8-1. if possible to do a fast acceleration on previous levels would that save time. or would it just not be possible at all.

you can do a fast acceleration on any level or room, but outside of 8-4 ILs, in 90% of the other cases it's not really important (unless you want to save some frames even though it won't save you another framerule).

I don't know much. But I do know that fase excelling on 1-1 doesn't save a FR.

Fast Accel is possible in every room, it just matters if it saves time or not.
The reason people do an accel in 8-1 is that you need to do the accel to save the TAS framerule. People do accels in 8-4 since it isn't affected by framerules, so doing them saves time.

In any%, those are the only 2 levels that are necessary to use a fast accel. Doing it in the other levels isn't needed since it doesn't save a framerule.

fast accel in every level save time, but not enough to save a new framerule.
in 8-4, every frame counts, so fast accel save time.
in 8-1 if you dont do a fast accel in the start and do a fpg you save 19 frames, with the fast accel in the start and do a fpg you save 21 frames, a new framerule.

It is possible

OK, but if you could do a fast acceleration in every room and save enough frames outside of the fast acceleration to somehow save a framerule, then would having a fast acceleration on every level be as important as the one on 8-1?

Well, if those precious frames would indeed save a framerule, then yes, it would be as important

I hope that it would eventually be possible to save some more frames in some levels so that this would be possible.

yea, it would start to be important then, it depends of it being needed for the framerule, as is something quite precise that only saves like 2-3 frames, and runners are not going to go for something like that if you can get the same time without doing so
indeed, any% is almost all perfect framerules as in TAS rn, and mostly is hard to spect new framerules saved at this point. in any% only the lightning 4-2 is left to equal TAS in framerules (and tho doing fast accel in that level is a diferent story)

Man, I really hope that we will find a faster way to hold R+B in 4-1 so that a fast accel will become necessary to save a framerule...