Evidence · document 23

Performance History

In the workspace: /performance-history/

Availability

Included on Pro and Advanced

This document is open to every reader on every plan. Availability tells you what a plan includes; it does not decide what you may read.

What it is for

Field Vitals answers what your visitors experienced this month. This answers whether it is getting better. The measurements are the same ones your own pages already send; what this adds is the axis nobody can see in a single window, because a site that is slow today and was slower last month is a site being fixed, and one number cannot say that.

How to use it

Fit the beacon in Field Vitals first. This page reads what that script collects and can invent nothing: with no beacon there is no history, and there is nothing to backfill.
Read the comparison at the top before the daily table. It answers the question you came with; the table below it is the evidence for that answer.
Check the visit count on each device before believing its change. A period with a handful of visits moves by twenty points per visitor, which is noise wearing a percentage sign.
Where a change is worth acting on, note the date it began and compare it against your own release log. This page can tell you when, never why.

Reading the result

Every figure is the share of measured visits that met the Core Web Vitals threshold, and a change is shown in points of that share, where up is always better. The averages beside it are context and are never differenced. Below twenty visits in either period the change reads "Not compared" with the two counts stated, because a number there would be read as a result and would not be one. A day absent from the daily table had no visits: nothing is interpolated, so a gap is a gap in your traffic rather than in the measurement.

What it cannot tell you

These are averages and shares, never percentiles. Every published Core Web Vitals threshold is defined on the 75th percentile, and this history is built from daily totals from which a percentile cannot be recovered - so the share of good visits is what is reported, counted against the real threshold at the moment each visit arrives. Ninety days is the whole history that exists: older records are deleted as they age past it, by the same design constraint that keeps no visitor identifiable. It cannot attribute a change to a cause, and it cannot see a page that does not carry the beacon, however slow that page is.

What it keeps

Nothing of its own. It reads the same daily aggregates Field Vitals collects and writes nothing back, so this page adds no record to the ninety days that were already being kept - and keeps nothing that identifies a visitor, because those aggregates never held it.