The Page speed box on SEO health says "No measurements yet". Do I need to run a test?

No test is needed. Page speed on Marketing → SEO Health is measured from real visitors' devices as they browse your store, because that is what search engines judge you on. A new or quiet store will show "No measurements yet" until enough visits come in.

Once data arrives, each measure shows a value with a rating of Good, Could be better or Too slow, and the box shows how many visitor measurements the figures are based on.

The Scan my store button (or Scan again) is separate: it checks your products, pages and blog posts for SEO problems and lists them with a score, filtered by Needs work, Errors only or All scanned.