That is to save 146KB on one image an 99KB on another, to save 2 seconds you'd have to using a 1mbps connection.

The first random internet article I pulled for real world cellular speeds suggests even unwired, people are getting 30mbps so that changes the article to:

Apple could load 66ms faster by adopting WebP

… but these are cached resources, so maybe…

Apple could perform the initial load 66ms faster by adopting WebP

… would be better.

Looking at the assets needed for an initial load, the fonts alone weigh in at about twice those images, so it probably wouldn't be noticeable.