optimize-gtm.js

// https://constantsolutions.dk/2020/06/delay-loading-of-google-analytics-google-tag-manager-script-for-better-pagespeed-score-and-initial-load/

document.addEventListener('DOMContentLoaded', () => {
  /** init gtm after 3500 seconds - this could be adjusted */
  setTimeout(initGTM, 3500);
});

document.addEventListener('scroll', initGTMOnEvent);
document.addEventListener('mousemove', initGTMOnEvent);
document.addEventListener('touchstart', initGTMOnEvent);

function initGTMOnEvent(event) {
  initGTM();
  event.currentTarget.removeEventListener(event.type, initGTMOnEvent); // remove the event listener that got triggered
}

function initGTM() {
  if (window.gtmDidInit) {
    return false;
  }
  window.gtmDidInit = true; // flag to ensure script does not get added to DOM more than once.
  const script = document.createElement('script');
  script.type = 'text/javascript';
  script.async = true;
  script.onload = () => {
    dataLayer.push({ event: 'gtm.js', 'gtm.start': new Date().getTime(), 'gtm.uniqueEventId': 0 });
  }; // this part ensures PageViews is always tracked
  script.src = 'https://www.googletagmanager.com/gtm.js?id=YOUR-GTM-ID-HERE';

  document.head.appendChild(script);
}