Real-time monitoring of Core Web Vitals via GA4 and GTM

Many companies optimize their site once and forget about technical metrics. But even a perfectly configured site loses performance over time - plugin updates, added scripts, new products or banners can completely break Core Web Vitals. That's why real-time monitoring is not a luxury, but a prerequisite for stable online sales. In this article, we will show how the integration of GA4, GTM and Core Web Vitals helped an online store identify and fix the problem before it affected its profit.

How a website lost performance — and no one noticed

This is an average Ukrainian e-commerce project with a catalog of over 1000 items. After the redesign, PageSpeed ​​indicators remained good, and everyone was sure that everything was working. But within a month, organic traffic began to fall, and the bounce rate increased. The site looked attractive, but mobile users began to abandon it within the first seconds.

The reason turned out to be unexpected — a marketing script that wasn’t optimized was stopping the page from rendering. Core Web Vitals were down, but no one noticed because the usual metrics (sessions, conversions, transactions) remained more or less stable.

Real monitoring: what the implementation of GA4 and GTM with Web Vitals yielded

After the analysis, we decided to integrate Core Web Vitals directly into GA4 analytics. Through Google Tag Manager, a special script was added to the site that reads three key metrics: LCP, CLS, and INP — and sends them as events directly to analytics.

Now the team could see in real time:

how quickly the main content (LCP) loads;

are there any "jumping" elements (CLS);

how quickly the site responds to user actions (INP).

Additionally, a dashboard was created in Looker Studio, which automatically showed average metric values ​​for each page and device.

What it gave to the business: analytics that prevents losses

A problem was discovered in the first week after the monitoring was launched: for one of the product categories, the average LCP was over 4 seconds on mobile. This was not immediately visible - the page looked fine, but users did not wait for the page to load completely and left.

After identifying this page, marketers excluded it from advertising campaigns, and developers optimized the banner loaded from an external source. LCP was more than halved, bounce rate decreased, and the page was returned to traffic.

Thanks to automatic data collection, the company received a weekly report on changes in Core Web Vitals and was able to make decisions quickly, not when traffic was already lost.

Constant control is not superfluous, but a strategic advantage

Integrating Web Vitals into GA4 via GTM allows companies to:

see the technical “temperature” of the site every day;

notice even small drops in performance before they affect sales;

differentiate metrics by device types, pages, or traffic sources;

automatically notify the team about deteriorating performance.

This creates a whole new level of control—with proactive rather than reactive actions. Companies can act proactively without losing revenue due to unnoticeable technical issues.

How does the company Glyanets implement such integration?

Gloss helps businesses not just launch a website, but keep it effective in the long term. In cases like this, the team:

Configures GA4 with support for custom events for Core Web Vitals;

Installs a special script via GTM that sends data to analytics;

Provides custom dashboards for real-time analysis;

Enables automatic alerts via email or Telegram;

Advises on how to correctly interpret this data and what actions to take.

Thanks to this approach, companies get not just a website, but a digital system that warns of problems before they become critical. This saves time, money, and most importantly, maintains customer trust.