GTmetrix vs Google PageSpeed: Which Should You Use to Measure Website Speed?

Website performance is a major factor in user experience and SEO, but how you measure that performance can make a big difference. Two of the most popular tools are Google PageSpeed Insights and GTmetrix. While both have their strengths, GTmetrix often offers a more useful and actionable perspective, especially for web designers, developers, and site owners who want to go beyond the surface.

So why do ads slow down a site? And why might GTmetrix be a better choice for diagnosing those issues? Let’s take a closer look.

Why Ads Slow Down Website Speed

Before comparing tools, it helps to understand a key cause of slow page loads: advertising scripts. Ads can significantly reduce your site’s performance — and not just because of file size.

Here’s what’s happening under the hood:

Third-Party Requests
Most ads are served from external sources (like Google AdSense or Mediavine), meaning your browser must connect to a separate server, download a script, and wait for that script to load before rendering the full page. These delays block your site’s initial paint and increase load time.

JavaScript Execution
Ad networks run complex scripts to determine which ad to serve, who’s viewing it, and how to track it. These scripts consume processing power and block other assets from rendering.

Heavy Media
Many ads include video or high-resolution images. These large files increase Largest Contentful Paint (LCP) and slow down the user’s perception of how fast your page loads.

Layout Shifts
Ads often load after your main content. This causes elements to shift around on the page, harming your Cumulative Layout Shift (CLS) — one of Google’s Core Web Vitals.

Unpredictable Performance
Since ads are served by third-party servers, your site’s speed becomes dependent on their performance. Even a well-optimized site can suffer if an ad server is lagging.

Measuring Site Speed: GTmetrix vs Google PageSpeed Insights

Both Google PageSpeed and GTmetrix analyze your website performance, but they do so in different ways and for different purposes.

Here’s why GTmetrix is often the better tool when you’re trying to understand and improve real-world performance.

Real Browser Testing vs Lab Simulations
GTmetrix uses an actual Chrome browser to load your site, simulating real-world behavior. It allows you to choose test locations around the world, connection speeds (3G, 4G, broadband), and device types (desktop, mobile). This provides realistic results you can trust.

Google PageSpeed Insights, on the other hand, uses a mix of simulated lab data and real user field data (when available). It’s valuable for understanding how Google perceives your site, especially for SEO — but it doesn’t let you test under different conditions or configurations.

Waterfall Charts and Asset-Level Detail
GTmetrix gives you an interactive waterfall chart, showing exactly how each file (script, image, style, etc.) loads — and when. You can see which third-party scripts (like ads, fonts, or analytics) are holding things up.

PageSpeed does not offer a visual waterfall, making it harder to diagnose what’s slowing your site down.

Historical Data and Tracking
When you sign up for a free GTmetrix account, it stores your test history so you can compare before and after optimization, monitor ongoing performance changes, and troubleshoot regressions after plugin updates or design changes.

Google PageSpeed does not offer historical performance tracking unless you integrate with other tools like Search Console or BigQuery.

Custom Test Settings for Developers
GTmetrix allows for advanced configurations, including turning JavaScript off, blocking specific scripts (great for testing site performance without ads), simulating throttled connections, and adding cookies or custom headers. This makes it a powerful diagnostic tool when you’re working on client sites or troubleshooting speed issues.

User Experience vs SEO Signals
PageSpeed Insights focuses on Core Web Vitals, which are important for SEO and ranking. But sometimes, PageSpeed penalizes necessary functionality — such as a video background, animation, or analytics scripts — that users actually expect.

GTmetrix emphasizes user experience, offering a more balanced and flexible view of performance. This makes it especially valuable when you want to optimize for real visitors, not just an algorithm.

So Which Tool Should You Use?

Here’s a side-by-side comparison:

GTmetrix vs Google PageSpeed Insights

Feature	GTmetrix	Google PageSpeed Insights
Uses real browser to test	✅ Yes	⚠️ Simulated (Lab Data)
Custom test location & speed	✅ Yes	❌ No
Waterfall view of assets	✅ Yes	❌ No
Historical tracking	✅ Yes	❌ No
Dev-level settings	✅ Yes	❌ No
SEO-focused scoring	⚠️ No	✅ Yes
Web Vitals reporting	✅ Yes	✅ Yes

Both GTmetrix and Google PageSpeed have their place — and ideally, you should use them together. PageSpeed gives you a snapshot of how Google sees your site in terms of SEO and compliance. GTmetrix gives you the full picture, helping you optimize performance from the perspective of the user.

If you’re managing a content-heavy site or running ads, GTmetrix will give you the insights you need to keep things running smoothly.