You misunderstood the whole purpose of the kata.

This should be first, other soultions aren't debugged, but rewritten.

@Gracjandogg You would want measurements taken from production app, leverage monitoring to spot major bottlenecks, and make room for adjustments if the product owner says so, or the performance impact is so much your app is bleeding money.

That's the difference between a senior and a junior. Everything is a tradeoff, and you're better off investing the next hour building stuff that give you ROI, and not just anything you can detect that can be done better.

If you're talking about hot paths? Yes, BUT you will also have to inline any loops, and other things that simply shouldn't exist in non-hotpath code. In sort: You're not wrong, you're just taking things to the absolute extremes by choosing to not use stuff the standard library provides because "slow".

I hate every kata where the question is more complicated than its solution...

"And what about the number 1010000000000000000000000000000000000000000000000000000000000000000 ?"

How often do you use that large number?