We can improve the webpage performance only loading the necessary css first (above the fold), and the rest afterwards. That result in the CSSOM finishing its load faster.

load-content-from-after-fold.html

<script type="text/javascript">
  const loadStylesheet = src => {
    if(document.createStylesheet)
      document.createStylesheet(src);
    else {
      const stylesheet = document.createElement('link');
      stylesheet.href = src;
      stylesheet.type = 'text/css';
      stylesheet.rel = 'stylesheet';
      document.getElementsByTagName('head')[0].appendChild(stylesheet);
    }
  }

  window.onload = function() {
    loadStylesheet('./style2.css');
  }
</script>