Google's John Mueller said on Twitter that "speed is complex & sometimes confusing" and there are so many tools and metrics out there that you can look at. He said that Google "focused on the Core Web Vitals, which have 3 main metrics." So maybe you should too?
John said "looking at these often help you find low-hanging fruit. One hard part is keeping in mind that you can do live ("lab") tests and use field data (from users), and they will show differences." John explained "Field data (like in PSI, Search Console, CrUx) takes longer to update, but shows what your users really see. It's a good first check. Lab data lets you quickly test changes as you make them (which plugin, setting, images, etc)."
Keep in mind that "one metric won't show you everything, but at least it can show you something," John added. "That might be better than being inactive because the numbers don't line up, but having a fuller picture can be useful if can act on the results shown," he explained.
Either way - I would not get bogged down on speed. Make sure your site loads reasonably fast and focus on your content.
Forum discussion at Twitter.
