Most of the time, performance tuning is about fixing problems. The site was slow, the checkout was unavailable, etc.

Like a doctor trying to diagnose an illness, we jump into a situation with little context, searching for clues to what could be causing the problem.

Metrics are tests that help narrow down causes. “They have a high LCP? How about their TTFB? Maybe it was a slow server causing high response times…”

The more you know about how the human body works, the more quickly you can identify the issue, the better you can prescribe a solution.

But we also have to prioritize. Often times we have limited resources and need to triage issues, deciding which issues to tackle first and how far to take them—or whether a fix is even possible. “They have a high TTI? How about their TBT? If FID is still OK, let’s focus on LCP first because we know that’s causing breakage in the funnel.”

Metrics are only half the story.

They’re diagnostics, not prescriptions.

Every metric needs to be interpreted and judged in context.