If you care about your website speed (and you should because Google has officially announced their Core Web Vitals are now a ranking factor), then you have almost certainly heard of using GTMetrix as a page speed testing tool to see how your website performs.
I have personally been using GTMetrix for years to test my website’s page speed and I constantly see GTMetrix reports shared in Facebook Groups dedicated to WordPress speed and in many blog posts.
In fact, there are several GTMetrix reports in many of my own blog posts on this site, Start Blogging 101.
The truth of the matter is that GTMetrix is really good for some things, but at the same time, it’s not so good for other things.
You should not be relying on GTMetrix for Core Web Vitals scores.
In this article, I’ll break down 5 reasons you should stop using GTMetrix for Core Web Vitals scores. Use the guide below to jump around as you need.
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1. GTMetrix Uses Outdated Google Lighthouse Versions
The first reason you should not use GTMetrix for testing Core Web Vitals scores is that GTMetrix uses outdated Google Lighthouse versions.
Google Lighthouse is the open-source technology that is used to measure the quality of web pages. It audits several factors on a web page including performance, accessibility, and SEO.
Google Lighthouse is what is being used in the background for several of the most popular page speed testing tools including Google PageSpeed Insights and GTMetrix.
Even though GTMetrix uses Google Lighthouse in the background, GTMetrix is slow to update to using the latest Google Lighthouse version.
What does this mean?
This means that if you test the same web page on Google PageSpeed Insights (which uses the latest Google Lighthouse version) and then test the web page in GTMetrix, you may see different results.
Rather than going into even more detail, it’s better to show you an example of this that I ran into while testing my own site.
Let’s break down the image above.
In this GTMetrix report of my home page on Start Blogging 101, you can see I had a Largest Contentful Paint of 637 ms, a Total Blocking Time of 118 ms, and a Cumulative Layout Shift of 0.8 which was in the red and causing a hit to my performance score.
However, when I tested the same exact page in Google PageSpeed Insights, I was passing all of Google’s Core Web Vitals including having a Cumulative Layout Shift of 0.
So, which page testing tool was correct? Google PageSpeed Insights or GTMetrix?
It turns out that GTMetrix was reporting incorrect data due to using an older version of Google Lighthouse.
As you can see in the image, the GTMetrix speed test was using Chrome version 86 and Google Lighthouse version 6.3.0.
The GTMetrix speed test report was taken in May of 2021 and the current Google Lighthouse version at the time was actually 7.3.0 which was a full major version ahead.
This means that GTMetrix was using a Google Lighthouse version that was already 8 months old.
Based on my observations, Google PageSpeed Insights gets updated to the latest Google Lighthouse version 2 weeks after a new version is released. GTMetrix, on the other hand, usually takes anywhere from 5-10 months to update to the latest Google Lighthouse version.
Therefore, if you are debating between using Google PageSpeed Insights or GTMetrix to test the speed of your website, you should absolutely be using Google PageSpeed Insights since it will be using the latest Google Lighthouse version and GTMetrix uses outdated Google Lighthouse versions.
2. GTMetrix Only Shows You Desktop Scores for Free
The second reason you should not use GTMetrix for Google Core Web Vital scores is that GTMetrix only shows you desktop Core Web Vital scores for free.
If you’ve ever tested your website’s speed using tools that show both desktop and mobile scores, the desktop version of your site will almost always score better than the mobile version.
However, between desktop and mobile, mobile is far more important to optimize your pages for because Google uses mobile-first indexing (more on this in the next section).
Even though GTMetrix uses Google Lighthouse in the background for determining Core Web Vitals scores, there are still some unique things that GTMetrix does on their end that can cause differences in scores between GTMetrix and Google PageSpeed Insights.
That being said, Google Lighthouse is an open-source technology which means it’s free for anyone to use and you can also hook into Google Lighthouse to test the performance of your pages programmatically for both desktop and mobile.
Therefore, it doesn’t really make sense for GTMetrix to require GTMetrix PRO just to see your mobile scores.
Is it worth paying for GTMetrix PRO in order to gain access to mobile Core Web Vitals scores? No. You can get both desktop and mobile scores for free directly from the source by using a number of Google tools such as Google PageSpeed Insights, Google Search Console, and the web.dev measure page.
3. Google Uses Mobile-First Indexing
The third reason you should stop using GTMetrix for Core Web Vitals scores is that Google uses mobile-first indexing.
As mentioned in the second point, GTMetrix only shows you desktop Core Web Vitals scores for free.
In order to get access to mobile scores, you have to purchase GTMetrix PRO which costs a minimum of $12/month for the lowest-tier plan. That’s a lot of money when you can get desktop and mobile scores for free with Google’s page testing tools!
What does mobile-first indexing mean? It means that Google primarily uses the mobile version of your pages for indexing and ranking.
Therefore, you should absolutely be focusing on optimizing your mobile scores rather than your desktop scores.
The problem with this is that it’s much harder to pass Google’s Core Web Vitals for mobile than it is for desktop.
It’s fairly easy to pass Google’s Core Web Vitals for your desktop pages.
If you aren’t passing Core Web Vitals for your mobile pages, then you should sign up for my 7 Days to WordPress Speed Mastery email course which is completely free and will show you exactly what you need to do.
4. GTMetrix Doesn’t Show You Real User Data
The fourth reason you should not use GTMetrix for Core Web Vitals scores is that GTMetrix doesn’t show you real user data.
Say your hosting server for your website is in Dallas, TX and you test your web page through GTMetrix from San Antonio, TX.
Well, you’re most likely going to see excellent Core Web Vital scores because you’ll have a lower TTFB (Time To First Byte) and a lower Largest Contentful Paint (LCP) due to testing your web page very close to your origin server.
However, what about real user data? What if an actual user visits your web page from outside of the United States far away from your origin server on a mobile phone with only 3G data? That user isn’t going to experience fast speeds anywhere close to what GTMetrix showed you.
After all, GTMetrix is merely simulating your scores very similar to how Google PageSpeed Insights shows “Lab data” which are simulated scores not based on real-world data.
This is why I always recommend using Google PageSpeed Insights which shows both simulated “Lab data” along with real-world “Field data” from real users.
Also, Google Search Console allows you to see your Core Web Vitals scores using the CrUX Report (Chrome User Experience Report). The CrUX report shows anonymized metrics from actual users that visit your site which is called field data.
This actual user data is what your website should be passing for Core Web Vitals in order to get a Google ranking boost. Unfortunately, GTMetrix does not show you actual user data, only simulated data.
5. GTMetrix Isn’t Google
The fifth reason you should stop using GTMetrix for Core Web Vitals is that GTMetrix isn’t Google (duh), and therefore doesn’t have any effect on whether you get a ranking boost from Google.
This reason seems pretty obvious, but there are many people I’ve seen that receive a good GTMetrix report where they pass all of Core Web Vitals and therefore expect that they’ll automatically receive a ranking boost from Google.
If your web page passes all Core Web Vitals when run through GTMetrix, you are not at all guaranteed to get a ranking boost from Google.
If your web page passes all of Core Web Vitals on GTMetrix, that’s a great start. However, there are still many factors that may prevent you from receiving a ranking boost from Google:
- Does the mobile version of your web page also pass Core Web Vitals according to Google?
- Does your web page pass Core Web Vitals based on actual real-world user data and not just simulated data?
- Does your web page pass Core Web Vitals according to the latest Google Lighthouse version?
What GTMetrix Should Be Used For
I just went over 5 reasons why you should not be using GTMetrix for Core Web Vitals scores. However, there are several things that GTMetrix should be used for.
Here are some items you should be using GTMetrix for:
1. Check Your TTFB (Time To First Byte)
The first item you should be using GTMetrix for is to check the TTFB (Time To First Byte) of your server based on various testing locations.
For example, say your website’s server is located in the United States. When you test your page speed with GTMetrix from San Antonio, TX, you can see a good indication of how fast your server is by looking at the TTFB which is the server response time.
It’s good practice to use Google Analytics to see where your website traffic is coming from. Is the majority of your traffic located in the same continent as your origin server? If so, that traffic is most likely going to experience a TTFB similar to your test results.
In my case, about 50% of my traffic is within the same continent as my origin server (United States), but the other 50% of my traffic is outside of the United States.
The traffic outside of the US experiences slower TTFB times (around 500-700ms) due to being farther away from my server in Chicago, Illinois.
Therefore, I decided to use Cloudflare APO (Automatic Platform Optimization) for $5/month which caches my HTML pages and serves them from Cloudflare’s edge servers.
This drastically cuts down on the TTFB time for users around the world and consistently gives users a TTFB of under 200ms no matter where they’re at in the world.
2. See a Waterfall Chart of All Files
The second item you should be using GTMetrix for is to see a waterfall chart of all the files (HTML, JavaScript, CSS, images, fonts) that loaded to display your page and how long they took to load.
You can use this GTMetrix waterfall chart to help diagnose certain issues that may be causing your website to load slowly.
For example, you may not know that you have 10 different font files loading on a web page or you may see files loading that you don’t think should be loading.
The waterfall chart will list out every file (HTML, JavaScript, CSS, images, and fonts) that is used to render your web page.
You can even filter down the file types to only show HTML, CSS, JS, XHR, fonts, or image files.
Each file request will also tell you how much time (in milliseconds) was spent blocking, sending, waiting, or receiving.
Understanding the GTMetrix waterfall chart and how files are loading on your web page can certainly help you diagnose and fix issues that are causing your web pages to load slowly.
3. Page Speed Improvement Suggestions
The third item you should be using GTMetrix for is to get some suggestions for items to improve your website page speed, although Google’s tools do this as well.
You can see in the example above, the web page I tested could be improved by using a CDN (Content Delivery Network) and by avoiding large layout shifts (which means a high CLS score).
These page speed suggestions can be expanded to give you more information on what is causing the issues.
GTMetrix can be used to help give you more information on page speed improvement suggestions, however many of these will also be the same as Google’s suggestions since GTMetrix is using Google Lighthouse in the background.
Conclusion
All in all, I want you to know that there’s nothing bad about using GTMetrix to test your website’s page speed. GTMetrix is a great tool for many reasons and is a tool I personally use for many situations.
Just know that you should not be using GTMetrix to solely rely on Core Web Vitals scores.
Getting good desktop scores in GTMetrix is fairly easy to do, but it doesn’t mean you’re all set to receive a Google ranking boost.
You also need to be passing Google’s Core Web Vitals on mobile (using Google PageSpeed Insights) and using real CrUX data which you can see through Google Search Console.
Did you learn something about GTMetrix today? What’s your preferred method of testing your website speed? Let me know in the comments below.
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Nice Post, totally agree.
For me I wouldn’t even use Gtmetrix for the 3 plus-reasons mentioned above.
1.) TTFB depends heavily on RTT’s (round trip times) which are completely different for desktop and mobile. Even on mobile those RTT’s differ a lot for G2, G3, G4 etc. so that the desktop TTFB (consisting of several RTT’s) tells you literally nothing – except you maybe have some good reference values for a typical good desktop TTFB at hand.
2.) To be able to troubleshoot a bad TTFB and bottleneck culprits, webpagetest.org’s free-to-use waterfall diagrams for mobile devices give much more useful and more precise information.
3.) As you say: dispensible, since already given by latest Lighthouse.
Cheers, Frank
Hey Frank, thanks for the comment! Some great points. I do like webpagetest.org’s waterfall diagram as well. Appreciate you reading!