I couldn't find the bit where the original performance claim was refuted (or not). Was it one of the listed options?
Which just does read/write - so it's the same as the "cat-simple" example, which is the slowest listed.
GNU cat [0] does copy_file_range if it can and falls back to a read/write loop otherwise, so it's unlikely to be much slower (possibly some overhead from argument parsing, but that's just a constant).
So the performance claims are wrong.
[0]: https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat...