I have experienced a Site Speed of currently showing Unavailable in Google Page Speed. It is showing that the data depends upon CrUX. Please make me understand why it is unavailable and what is CrUX
Google released an update to Pagespeed insights recently, bumping it from version 2 to version 4. (https://developers.google.com/speed/docs/insights/release_notes)
The new version includes data from the Chrome User Experience Report (CRUX), which is the speed score. You can see how that is calculated here:
https://developers.google.com/web/tools/chrome-user-experience-report/
On my two sites I have tested, the speed score is unavailable. For a large site like the BBC, there is a score available:
https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fwww.bbc.co.uk
I'm guessing that the new site speed score is either rolling out gradually, or requires a certain volume of visitors to activate.
The old optimisation score is still there, but seems to be giving slightly different results for my sites, presumably they have updated a few of their testing metrics to reflect advances in web technologies, although that is just speculation on my part.