body-start-script.js

<script>
  let temp = document.createElement("div");
  temp.innerHTML = document.getElementById('shopify_header_scripts').innerHTML
    .replace('</head>', '');
  let elements = temp.children;
  let lazyElements = [];

  // Loop through each element
  for (var i = 0; i < elements.length; i++) {
      // Create a new element
      let newElement = document.createElement(elements[i].tagName);
      // Get all the attributes of the element
      let attributes = elements[i].attributes;
      // Insert the innerHTML of the element
      if (elements[i].innerHTML) {
          newElement.innerHTML = elements[i].innerHTML;
      }
      // Loop through each attribute
      for (let j = 0; j < attributes.length; j++) {
          let attribute = attributes[j];
          newElement.setAttribute(attribute.name, attribute.value);
      }

      if(newElement.tagName === 'SCRIPT') {
        lazyElements.push(newElement);
      } else {
        document.head.appendChild(newElement);
      }
      
  }

  let lazyLoadShopifyScripts = function() {
    lazyElements.forEach(function(element) {
      document.head.appendChild(element);
    });
  }

  document.body.addEventListener('touchstart', lazyLoadShopifyScripts, { once: true });
  document.body.addEventListener('mouseover', lazyLoadShopifyScripts, { once: true });
</script>

theme-header.liquid

<!-- This snippet tricks Shopify into injecting their end of head tags and lets me parse and re-inject it -->
  <noscript id="shopify_header_scripts">
    </head>
  </noscript>
</head>

theme.liquid

{% comment %}{{ content_for_header }}{% endcomment %}
{% capture header_scripts %}
  {{ content_for_header }}
{% endcapture %}

{% assign header_scripts = header_scripts | replace: 's.src = urls[i]', 'if(!/^\?/.test(urls[i])) s.src = urls[i] ' %}
{% assign header_scripts = header_scripts | replace: "window.addEventListener('load', asyncLoad, false)", "document.body.addEventListener('touchstart', asyncLoad, { once: true }); document.body.addEventListener('mouseover', asyncLoad, { once: true });" %}
{{ header_scripts }}