There's definitely a conversion cost. For strings, Python apparently caches the UTF-8 encoded string, so if you _repeatedly_ transfer it to Rust I suspect (but haven't checked) that the cost is much lower.

In general I suspect it's the usual "NumPy arrays are fast, everything else you better be getting a sufficiently large boost from the low-level code to justify conversion".

For the thing I prototyped in Rust, it was wrapping the `ahocorasick` crate which was in fact faster than `pyahocorasick` which is written in C or Cython or something. Both have similar conversion costs, probably, so it came down to "for lots of data the Rust version was faster".