Where did you get the two machine instructions number? A swap could easily be a exchange of e.g. 16 (the size of a slices &[T]/&str on a 64-bit computer) or 24

Where did you get the two machine instructions number? A swap could easily be a exchange of e.g. 16 (the size of a slices &[T]/&str on a 64-bit computer) or 24 bytes (Vec<...>/String) (or larger), and there's no way to use single SIMD instructions for these in general. Also, "per operation" is vague, since the swaps would have to happen repeatedly in a loop, so a single .insert call is likely to result in multiple swaps.