Two of the numbers Google grades a site on are how long the biggest thing on the screen takes to turn up, and how long the page takes to answer when somebody taps something.
You could always measure both from inside the page, as long as the visitor was in Chrome. Safari didn't report either. On a site where half the visits come from iPhones, that's half the audience you know nothing about.
What changed
Safari does both, as of December. The same code that was already gathering them from Chrome picks them up.
The third score, the one for things jumping about while the page is still loading, is still Chrome only. So there's a gap left.
Still worth turning on. Testing it yourself on your own machine tells you what your own machine thinks, and I've rarely found that matches what visitors get on an older phone and a worse connection.