Here's a benchmark over time of jxl-rs (Rust) vs libjxl (C++) across both memory usage and performance: https://jxl-rs-perf.lucaversari.it/. I'm inclined to trust that it's a decent benchmark since it's from Luca Versari, a key person in the original JPEG XL standardization effort and development of libjxl, and is now the main developer of jxl-rs.

Seems to be between a little and a lot faster in almost all of the tests.