Jonathan, thanks for linking to some of the key points. To elaborate a bit on what you posted...
Here's the PageSpeed test page, with a blank URL input field for members here to post their own urls. (For everybody's protection on WebmasterWorld, we don't publicly discuss test results on personal sites, so I removed the sample url posted.) PageSpeed Insights https://developers.google.com/speed/pagespeed/insights/
As I read the additional documentation, "Speed" is obtained in PageSpeed Insights (PSI) by comparison with various datasets, browser info, etc.... and depending on the circumstances of your test (desktop, mobile, prototype, etc), the data may not be available directly. Thus, the "Unavailable" tab. In that case, Google has created other performance auditing tools to help you obtain useful performance indications and benchmarks in other ways.
This paragraph in the FAQ that was linked to in the OP is most relevant to that and leads to some important information. (On this forum, to preserve the hashtag as a named anchor, copy and paste it into your browser's address bar).... Google PageSpeed Insights - FAQ - Speed Score Uavailable https://developers.google.com/speed/docs/insights/faq#speedscoreunavailable
Why am I not seeing any speed data for a URL?
PSI uses data from the Chrome User Experience Report [developers.google.com], which provides speed data for popular URLs that are known by Google's web crawlers. If the speed data for the queried URL is not available in the CrUX dataset, we recommend using Lighthouse [developers.google.com] to run a synthetic performance audit to estimate page speed, and investigate page optimization recommendations provided by PSI and Lighthouse.
I find the sources of data and the methodology particularly interesting, and, at my current stage of knowledge, challenging...
The Chrome User Experience Report is powered by real user measurement of key user experience metrics across the public web, aggregated from users who have opted-in to syncing their browsing history, have not set up a Sync passphrase, and have usage statistic reporting enabled....
Google's Lighthouse appears to be a major tool unto itself, compatible with a lot of building blocks that Google is providing.
Lighthouse is an open-source, automated tool for improving the quality of web pages. You can run it against any web page, public or requiring authentication. It has audits for performance, accessibility, progressive web apps, and more. Lighthouse https://developers.google.com/web/tools/lighthouse/
You can run Lighthouse in Chrome DevTools, from the command line, or as a Node module. You give Lighthouse a URL to audit, it runs a series of audits against the page, and then it generates a report on how well the page did....
I've thus far only gotten about 1/3 of the way through the 37-minute video introduction, which I strongly recommend as an introduction to the tool.
Potentially much to be discussed.