Wait no, I want to spend months figuring out how to optimize user login for my 70 users to go from 20ms to 12ms.

I think user login is one of the few things in your application that is better if it takes longer (e.g. stronger password hashing).

Well, do you see programming as an obligatory tool to make stuff, or do you aspire to expertise and mastery of the craft?

I think those are both probably legitimate approaches, but if it's the latter you really want to understand how the hardware works.

It's fine for a side project, but a waste of time for a business. I'm not going to charge my employer or client hundreds of extra hours just so I can learn a cool new thing.

I too have side projects :)

It doesn't apply to 99% or maybe 99.9% of tasks, but given enough development time, it applies to most business applications eventually.