Personally, in an application, I would quickly start bottom up. It's unlikely, but maybe there's an obvious way to improve one of those functions. An unnecessar

Personally, in an application, I would quickly start bottom up. It's unlikely, but maybe there's an obvious way to improve one of those functions. An unnecessary copy or similar can easily happen. Then, yes, spend time bottom up, what's were you're more likely to find consistent gains, usually by finding ways to call those low level functions less often.