I don't understand your benchmark. Do you mean that os.walk() is slower with fast_bottom-up.patch because islink() is called or because I replaced "for entry in scandir(top):" with "entry = next(scandir_it)"?

Are you testing the top-bottom or bottom-up?

Here is a variant of my patch with "for entry in scandir(top):". I would prefer to avoid this variant with a boolean to not catch OSError on the recursive call to walk() if next() has similar performances.