We're not talking about writing assembly by hand here. If your software has a million daily users and wastes a minute of their day, that's about 9 work-years of labour wasted every single day.

In a 5-year lifecycle that's about 10,000 years of human labour wasted. Yes, I had to quadruple-check this myself.

Does it take 10,000 work-years of effort, per project, to train its developers to write reasonably performant code?

Of course not all of this would translate into actual productivity gains but it doesn't have to.