Use Service Worker to load fonts async/direct via client-side logic.

sw-font-load.js

// Load fonts async by default
// Load sync from SW cache when available.
//
// Snippet assumes your Service Worker caches fonts as part of installation, so that
// an 'activated' Service Worker means the fonts are cached with 100% confidence.
if (
  'serviceWorker' in navigator &&
  navigator.serviceWorker.controller !== null &&
  navigator.serviceWorker.controller.state === 'activated'
) {
  // Directly load fonts from cache. File contains inlined fonts.
  document.write('<link rel="stylesheet" href="/css/fonts.min.css" type="text/css">');
  
  // Skip the font-observer
  document.documentElement.classList.remove('wf-loading');
  document.documentElement.classList.add('wf-oswald');
} else {
  // Load non-blocking by default, font-observer will handle class switching later on.
  loadCSS('/css/fonts.min.css');
}