[Byamarro]

I think that early in development you should be able to spam a lot of hypothesis and quickly test them and check how people interact with your software. Whether your software makes sense is more important than whether it's fast.

People are also highly unpredictable, so it is usually a matter of trial and error, very often their feedback may completely erase wide sets of assumptions you were building your product around.

It's borderline impossible to do it on mature product, but rewriting mature product to something faster is not borderline impossible - it's just very hard.

Note that it doesn't apply if you just program something in accordance from an rfc where everything is predefined.