`structuredClone` is ~2.4× slower than a JSON round-trip

A JSON round-trip is faster for deep copies — but it destroys Date, Map, undefined, and cyclic references. Do not pick it on speed alone.

Result

Run	`structuredClone`	`JSON.parse(JSON.stringify())`	Ratio
Run 1	9.697	3.560	2.72×
Run 2	9.362	3.851	2.43×
Run 3	8.586	3.949	2.17×

Median spread: `structuredClone` CV 5.1% / `JSON.parse(JSON.stringify())` CV 4.4% / Ratio CV 9.2% (computed from the 3 runs)

Conditions

Deep-copying a ~1 MB, 4,000-record object (the JSON path re-parses the same string).

Where this number does not apply

We measured speed only, not memory. The JSON path allocates a large temporary string, which can hurt on big payloads.

Reproduce

cd bench
node --expose-gc micro-fixed.mjs --out ../data/run.json --samples 21

Run it three times on the same machine; if the spread of median stays under 10%, you should land on the same conclusion. If your numbers disagree, tell us and we will retract.