How much is Google Tag Manager
slowing down your site?
Enter your URL and we'll run a Lighthouse test with and without Google Tag Manager. You'll get a report showing the exact performance cost of your GTM container — delivered to your inbox.
Get your free GTM impact report
Results are emailed to you — typically within 5 minutes.
Enter the page URL to test
We'll test the exact URL you enter — make sure it's the page you want to analyse.
Test submitted
We're running two Lighthouse tests on your page now.
Your report will be emailed to within 5 minutes.
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How this test works
Baseline test
We run a Lighthouse audit on your page using mobile emulation with simulated 4G throttling — the same conditions Google uses for PageSpeed Insights.
GTM-blocked test
We run the same test again, but block all requests to googletagmanager.com. This prevents GTM and every tag it manages from loading.
Compare the difference
We compare Performance score, Total Blocking Time, Time to Interactive, LCP, FCP, CLS, and Speed Index between the two tests. The delta is the cost of your GTM container.
Get your report
A branded report is emailed to you with the full comparison, colour-coded deltas, and a plain-language verdict on how much GTM is affecting your page.
Why GTM impacts page speed
Google Tag Manager is a lightweight container — the script itself is small. But the tags it loads are not. Analytics, ad pixels, A/B testing tools, heatmaps, consent managers, and retargeting scripts all execute JavaScript on your visitor's main thread.
On mobile devices with limited CPU, these scripts compete for execution time. The result is Total Blocking Time (TBT) — periods where clicks, taps, and form interactions go unacknowledged because the browser is busy running tag JavaScript.
TBT is a lab metric, but it correlates directly with INP (Interaction to Next Paint), which is a Core Web Vital measured on real users. High INP means slow interactions — and Google uses it as a ranking signal.
Common GTM performance culprits
Frequently Asked Questions
How much does Google Tag Manager slow down my website?
GTM itself is lightweight, but the tags it loads (analytics, ads, A/B testing, consent managers) can add significant main-thread blocking time. On heavy tag setups, we've seen GTM containers add 1-2 seconds of Total Blocking Time and delay Time to Interactive by 5-10 seconds on mobile.
How does this test work?
We run two Lighthouse tests on your page using mobile emulation with simulated 4G throttling. The first test loads your page normally. The second blocks all requests to googletagmanager.com, preventing GTM and every tag it manages from executing. The difference shows the performance cost of your GTM container.
What is Total Blocking Time (TBT)?
TBT measures the total time the browser's main thread is locked by JavaScript for more than 50ms between First Contentful Paint and Time to Interactive. High TBT means clicks, taps, and form interactions go unacknowledged during page load. It accounts for 30% of the Lighthouse Performance score.
Does removing GTM improve Core Web Vitals?
Removing GTM reduces main-thread blocking (TBT/INP) but typically does not affect LCP or CLS. The real-world impact depends on how many tags your container loads. CWV is measured using field data from real users (CrUX), not lab tests.
Is server-side GTM (sGTM) faster?
Server-side GTM moves tag execution from the user's browser to a server, eliminating main-thread blocking. This typically reduces TBT significantly. However, some tags (consent managers, A/B testing) must remain client-side.
How long does the test take?
Each Lighthouse test takes about 30-45 seconds. Since we run two tests (baseline + GTM-blocked), expect your report within 5 minutes of submitting.
Statistical Significance Calculator — check if your A/B test results are real
Need the full picture?
Tag-by-tag performance audit
We'll analyse every tag in your GTM container individually, rank them by impact, and give you a prioritised action plan.