Using Web Workers

You can run highlighting inside a web worker to avoid freezing the browser window while dealing with very big chunks of code.

// In your main script:
addEventListener('load', () => {
  const code = document.querySelector('#code');
  const worker = new Worker('worker.js');
  worker.onmessage = ({data}) => { code.innerHTML = data; }
  worker.postMessage(code.textContent);
});

// In worker.js:

onmessage = (event) => {
  importScripts('<path>/highlight.min.js');
  const result = self.hljs.highlightAuto(event.data);
  postMessage(result.value);
};