Overall user experience, mobile battery life, and many other metrics are really hard to fix by micro-optimizing a few functions. The key to a system that doesn't feel sluggish is being conscious of performance issues when making design decisions.

This pendulum swings back and forth, and we went from "every bit counts" madness of the early days, to the polar opposite of "just burn cycles, whatever".

Systems where every interaction feels sluggish are a pain to use, and often nearly impossible to refactor for better performance.