`for…in` is 17% slower than `Object.keys`

To enumerate keys, Object.keys is faster. for…in also walks the prototype chain, and that costs extra.

Result

Run	`for…in`	`Object.keys` + array loop	Ratio
Run 1	31.349	27.763	1.13×
Run 2	31.549	26.959	1.17×
Run 3	32.504	27.044	1.20×

Median spread: `for…in` CV 1.6% / `Object.keys` + array loop CV 1.3% / Ratio CV 2.6% (computed from the 3 runs)

Conditions

Enumerating an object with 100,000 keys.

Where this number does not apply

17% is small enough to disappear behind real work. Choosing for…in for readability is often the right call.

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.