[recursive]

> array.unshift(x), is mutating and slow

This year it's slow. I wouldn't count on that being true in five years. I mean, it might, it might not. There's a risk in optimizing too much for current conditions. You can easily end up over-fitting for the current set of coincidences, and end up with less readable code that's actually slower in the long run.

But by all means, measure and improve until it's fast enough.