A few opinions,
written into the code.
Plenty of tools will hand you a performance score. Very few will tell you when that score should not be trusted. VitalLens takes positions on which data gets to decide, what counts as a real change, and what is worth waking someone up for — and then behaves that way.
Real visitors decide
The verdict on a page comes from field data or it does not come at all. When Google’s CrUX has seen too little traffic to report on your URL, we withhold the verdict rather than quietly substitute a lab number that was never about your audience.
A lab run is a diagnosis
Lighthouse is superb at telling you which element painted last and which request held it up. It is one load on one simulated phone, so it gets to explain the problem — it does not get to grade you.
Noise is not news
Monitoring ignores movement under half a percent outright, and only writes to you once a metric worsens past the threshold you set. An alert you have learned to swipe away is worse than no alert at all.
We use Google’s thresholds, unmodified.
The same boundaries apply to field and lab values through one shared function, so a number never changes meaning depending on which screen you are looking at. If your page passes here, it passes where it counts.
| Metric | Good | Needs work | Poor |
|---|---|---|---|
| Largest contentful paint | ≤ 2.5 s | 2.5 – 4.0 s | > 4.0 s |
| Interaction to next paint | ≤ 200 ms | 200 – 500 ms | > 500 ms |
| Cumulative layout shift | ≤ 0.10 | 0.10 – 0.25 | > 0.25 |
| First contentful paint | ≤ 1.8 s | 1.8 – 3.0 s | > 3.0 s |
| Time to first byte | ≤ 800 ms | 0.8 – 1.8 s | > 1.8 s |
People who have to answer for the number
Engineers who need the failing element named, not a list of audits to read through.
Performance leads who have to choose what the team does this sprint and defend it afterwards.
Growth and SEO teams who watch vitals because Google does.
Agencies and founders who need a report a client will actually read.
Easier to judge from a report than a manifesto.
Run one on a page you already worry about.