"That costs ~5 ns when the line is cold"

I don't see how that could possibly be true. Sounds like a low-ball estimate.

Also i wish to point out that the "tcmalloc" being used as a baseline in these performance claims is Ye Olde tcmalloc, the abandoned and now community-maintained version of the project. The current version of tcmalloc is a completely different thing that the mimalloc-bench project doesn't support (correctly; I just checked).