Webpages can be good citizens of battery life. It’s important to measure the battery impact in Web Inspector and drive those costs down. Doing so improves the user experience and battery life.

[Apart from 2 very specific cases where treatments needed to be done without the user request] I have seen exactly zero other instances where Web Workers would have improved the user experience.

Cumulative Layout Shift (CLS) is a relatively new metric to help you understand the delightfulness of the user experience.