Page performance is a non-interaction event that measures Core Web Vitals metrics and page load time. It is usually fired a couple times while the webpage finishes loading to optimize performance. Not all metrics / parameters are present with all events, whenever a couple of them are available, the event is being fired. Thanks to the implementation, timings are counted from the first milliseconds when the webpage starts to load, even from before the GTM container was loaded into the page’s context, thus they provide accurate measures regarding how your page performs.

The best about this implementation is that contrary to most Web Vitals validation tools out there this solution does not use a test server with test browsers set up for this very purpose to measure how your page performs, but provides data on the actual load time metrics for each individual user that loads your webpage and enables you to narrow in on possible bottlenecks right where they might occur.

Core Web Vitals metrics measured

Largest Contentful Paint (LCP)
First Input Delay (FID)
Cumulative Layout Shift (CLS)
First Contentful Paint (FCP)
Interaction To Next Paint (INP)
Time To First Byte (TTFB)

You may read further on what each of these metrics exactly means on web.dev by Philip Walton.

Parameters

page_load_time: The complete time untill the page fully finishes loading (window load)

web_vitals_lcp_sample_id: A unique measurement ID to associate multiple LCP measurements with a single page load.

web_vitals_lcp_sample_value: Value in milliseconds.

web_vitals_lcp_sample_delta: Delta to the previous LCP measurement on the same page.

web_vitals_fid_sample_id: A unique measurement ID to associate multiple FID measurements with a single page load.

web_vitals_fid_sample_value: Value in milliseconds.

web_vitals_fid_sample_delta: Delta to the previous FID measurement on the same page.

web_vitals_cls_sample_id: A unique measurement ID to associate multiple CLS measurements with a single page load.

web_vitals_cls_sample_value: Value in milliseconds.

web_vitals_cls_sample_delta: Delta to the previous CLS measurement on the same page.

web_vitals_fcp_sample_id: A unique measurement ID to associate multiple FCP measurements with a single page load.

web_vitals_fcp_sample_value: Value in milliseconds.

web_vitals_fcp_sample_delta: Delta to the previous FCP measurement on the same page.

web_vitals_inp_sample_id: A unique measurement ID to associate multiple INP measurements with a single page load.

web_vitals_inp_sample_value: Value in milliseconds.

web_vitals_inp_sample_delta: Delta to the previous INP measurement on the same page.

web_vitals_ttfb_sample_id: A unique measurement ID to associate multiple TTFB measurements with a single page load.

web_vitals_ttfb_sample_value: Value in milliseconds.

web_vitals_ttfb_sample_delta: Delta to the previous TTFB measurement on the same page.

Utilization