What are you talking about. The time complexity doesn't change by moving everything into the reducer, it's the space complexity that changes because .map creates a new array. It's not 1ms vs 3ms...
You are right that it doesn't take three times as long, my mistake.
But it's never just "1+1+1=3" either.
Especially if the arrays are large, and the operations are simple, then the overhead of copying the array doesn't just affect memory use, it also takes longer.
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