Windows NT on 600MHz machine opens apps instantly. What happened?

Misunderstood or intentionally misapplied "wisdom" like "premature optimisation is the root of all evil".

Programmer comfort, unified frameworks, higher level languages over user experience.

Focus on end users instead of professional users.

Stupider programmers and programming through "good enough", "everyone can code" and "salaries are too high, hire someone cheaper".

Computers getting cheaper, therefore users buying new machines to run software faster, instead of programmers trying to get stuff running fast on current hardware.

Yes, and there is never, ever time to go back and address the weaknesses of the initial fast and dirty approach. If you prove out a product, it's proved, and any further work addressing tech debt is pure overhead, from the perspective of the business. Like so many problems, the ultimate solution is large scale collapse. We are now open to a super-virus that renders all of our (internet connected) compute worthless, or worse. It's a good opportunity to learn a trade, like carpentry, pottery or (pre-2005) auto repair.