Findings

All six survived three independent runs with the same direction and roughly the same magnitude.

• `+=` is 3.6× faster than `push + join` for string building
• `startsWith` is 3.9× faster than a `/^s1/` regex
• `for…in` is 17% slower than `Object.keys`
• `Float64Array` sums about as fast as a plain array (6% gap)
• `Array.includes` is ~28× slower than `Set.has`
• `structuredClone` is ~2.4× slower than a JSON round-trip
• `JSON.stringify` is ~2.4× faster than `JSON.parse`
• A spread copy is ~1.4× slower than `Object.assign`
• `toFixed` is ~1.6× slower than `Intl.NumberFormat`
• Sorting numbers is ~1.4× slower than sorting strings