LargestContentfulPaintBookmarklet.js

/**
 * A bookmarklet for viewing the largest contentful paint in a page.
 * Will show each LCP after the bookmarklet is clicked.
 *
 * To install:
 * 1. Copy the code starting from the line beginning `javascript:`
 * 2. Add a new bookmark in Chrome, and paste the code in as the URL.
 **/
javascript:(function(){
  try {
    const canvas = document.createElement('canvas');
    canvas.style.width = '100%';
    canvas.style.height = '100%';
    canvas.width = window.innerWidth;
    canvas.height = window.innerHeight;
    canvas.style.position = 'absolute';
    canvas.style.left = 0;
    canvas.style.top = 0;
    canvas.style.zIndex = 100000;
    canvas.style.pointerEvents = 'none';
    document.body.appendChild(canvas);
    const context = canvas.getContext('2d');
    const po = new PerformanceObserver((list) => {
      for (const entry of list.getEntries()) {
        let rect = entry.element.getBoundingClientRect();
        context.clearRect(0, 0, canvas.width, canvas.height);
        context.strokeStyle = 'red';
        context.strokeRect(rect.x,
                           rect.y,
                           rect.width,
                           rect.height);
      }
    });

    po.observe({type: 'largest-contentful-paint', buffered: true});
  } catch (e) {
  }
})();

Awesome! Thanks!

Very useful!
You can also select and drag the code into the bookmarks tab, then click on the bookmark.