`+=` is 3.6× faster than `push + join` for string building

When you join 10,000 short strings, building with += beats collecting into an array and joining. V8's cons strings make repeated concatenation cheap.

Result

Run	push + join	`+=` concatenation	Ratio
Run 1	0.300	0.084	3.57×
Run 2	0.323	0.090	3.58×
Run 3	0.308	0.087	3.55×

Median spread: push + join CV 3.1% / `+=` concatenation CV 2.9% / Ratio CV 0.3% (computed from the 3 runs)

Conditions

Joining 10,000 segments ('segment-0' … 'segment-9999') into one string.

Where this number does not apply

The gap shrinks for multi-megabyte strings or when you concatenate only a few times. Other browsers may behave differently.

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.