A 100 PageSpeed score does not contain INP, and that is the most expensive thing about it
A 100 in PageSpeed Insights is one of the very few numbers in this job you can chase to a literal maximum, which is exactly why people chase it. It is a score, not a feeling, and getting it forces real fixes. It is also a lab score: one simulated load, on a fixed device and network profile, run once. Two of the three things that most determine whether a site feels fast to a real person are not in…
A score of 100 on PageSpeed Insights is one of few numbers you can chase to a literal maximum, driving people to make real fixes. However, this score is a lab score, not a feeling, and two out of three factors that determine site performance are not included. It's a weighted average of five metrics, with Total Blocking Time (TBT) carrying three times the weight of First Contentful Paint (FCP).
This is why two sites with identical scores can feel fast at different times. Each metric has its own scoring curve, with the relationship between time saved and points gained becoming less predictable beyond 96. The main thread's blocking is the biggest issue, often caused by a dozen small tasks like parsing and executing bundles, loading third-party scripts, and executing analytics tags.
These issues are rarely due to slow code but rather sequencing. The score can also vary due to various factors like A/B tests, internet routing, and browser extensions, making a single number an unreliable indicator of performance. Interaction to Next Paint (INP) is not included in the performance score, making it an essential metric that lab scores cannot provide.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
