Here's another way of looking at it. As long as you're below a certain threshold, users would much rather you implement 5 new features than spend that time squeezing 100ms on the load time.

Another related side-reason why it's slow is we use higher and higher levels of abstraction, exactly to increase development velocity and be able to add more features quicker. I could write a native app in pure assembly and have it be blazing app, or I can write a webapp on top of web frameworks running in Electron, but in a fraction of the time. As long as my app is usable, I'll get all the user while the other person is still trying to finish their app.