Jitter says something about (in)consistency of time periods or intervals. It will be a function of scheduling decisions, not the overhead of a single call.

In Nix, on an AP core, sure, because there isn't any scheduling. When there is scheduling of any sort,

it's the scheduling that results in the jitter, not either of these two time implementations, surely.

You could clearly say "faster", but I'm not convinced about "jitter-free". For instance, you can still be pre-empted for variable

time between the invocation of "nsec", its arrival in the kernel, and on return from the kernel before and after return from "nsec".

Preventing that is a scheduling matter, with both implementations.