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interface_lag_detector.js

const LagDetector = (() => {
  let run = true;
  const TRESHOLD = 1e3/60 + 1; // ~60 FPS
  let lastTime = +new Date();
  (function loop() {
    const time = +new Date();
    const diff = time - lastTime;
    if (diff > TRESHOLD) {
      console.log(`Interface blocked for ${diff}ms`);
    }
    lastTime = time;
    run && setTimeout(loop, 1);
  })();
  return { stop: () => run = false };
})();

const simulateRandomLags = () => {
  const whenDelay = () => Math.trunc(Math.random() * 8e3);
  const lagDuration = () => Math.trunc(200 + Math.random() * 500);
  const lagLoop = () => {
    const t1 = Date.now();
    const duration = lagDuration();
    while (1) {
      const t2 = Date.now();
      if (t2 - t1 > duration) break;
    }
    setTimeout(lagLoop, whenDelay());
  };
  lagLoop();
};

simulateRandomLags();