A little wrapper function for detecting and displaying jank. Hacked together for a demo, may not be suitable for any legitimate purpose. :-)

jank.js

const revealJank = (fn) => {
    // jank is a master on/off switch from outer scope
    if (!jank) return fn();

    // scrim is visible when .janking is on body
    document.body.classList.add('janking');
    setTimeout(() => {
        fn();
        let lastFrame = window.performance.now();
        let goodFrames = 0;
        function check() {
            const thisFrame = window.performance.now();
            const elapsed = thisFrame - lastFrame;
            if (elapsed < 64) {
                if (goodFrames++ > 2) {
                    document.body.classList.remove('janking');
                    return;
                }
            }
            else {
                goodFrames = 0;
            }
            lastFrame = thisFrame;
            window.requestAnimationFrame(check);
        }
        window.requestAnimationFrame(check);
    }, 0);
}