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I'm working on improving the performance of a site. After some investigation, I'm focusing on reducing the Total Blocking Time (TBT). Chrome Lighthouse tells me to "Reduce the impact of third-party code Third-party code blocked the main thread for 250 ms". It seems that Google Tag Manager and Google Analytics are blocking the thread for most of the time:
Checking the performance tab confirms this too: I have 4 "long-tasks" and 3 out of them are related to Google Tag Manager or Analytics.
The below code shows, how Google Tag Manager is included in the site:
<head>
<!-- Google Tag Manager -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl+ '>m_auth=XXXXXXXXXXXXX>m_preview=env-2>m_cookies_win=x';f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXXXXX');</script>
<!-- End Google Tag Manager -->
Is this normal that GTM has 3 tasks and blocks the main thread and causes a high TBT? Am I doing anything wrong? Is there any way to fix this and reduce the TBT while having GTM on the site?
Thanks! W.
250
asked Dec 18 '25 23:12
The best solution I've found is partytown, which moves your analytics scripts off the main thread and into a worker
117
We solved it in the following way, keep in mind that you will lose the stats of people who spend less than 5 seconds viewing the page.
window.addEventListener('DOMContentLoaded', (event) => {
window.onload = function() {
setTimeout(function() {
(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl+ '>m_auth=XXXXXXXXXXXXX>m_preview=env-2>m_cookies_win=x';f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXXXXX');
}, 5000);
};
});
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