I assume you refer to https://github.com/torbiak/git-autofixup. I have also used it, and its ok but not perfect.
> it doesn’t say anything when there was no match
that's what it should do
> it can be quite slow as things grown
How? All the slowness (on large repos) I've seen has been fixed.
No it is not.
> How?
I don’t know, that’s just an observation from using it, semi regularly I autofixup changes and it takes a while to do anything.