Finding the Last Bottlenecks in json2xml with Flamegraphs

I released json2xml-rs 0.4.2 and json2xml 6.5.0 after a focused profiling session across both implementations. The final result: Pure Python conversion improved from 83.0 ms to 57.2 ms, a 31.1% reduction. The Rust accelerator improved from 6.007 ms to 5.632 ms, a further 6.23% reduction. Both implementations produced identical XML before and after the changes. The Python suite reached 100% statement coverage: 421 tests covering all 762 statements. The Rust release shipped first, followed by the Python wrapper that requires it. This post documents not only the code that survived, but also the prompts, rejected experiments, review feedback, release failures, and verification work that got it over the line. How the session started My first prompt was intentionally outcome-focused: I want to push a bit more optimisation. Let’s use flamegraph to find the bottleneck and implement fix. Do it from a new branch and use latest python 3.15 beta to do so. Use uv to manage python. That established…