key RUM perf metric: time-to-first-jank

firstJank.js

if (window.requestAnimationFrame) {
  function checkForJank () { // eslint-disable-line no-inner-delcarations
    const frameStart = Date.now()
    window.requestAnimationFrame(() => {
      const frameDuration = Date.now() - frameStart
      if (frameDuration > 17) {
        Perf.log(FirstJank)
      } else {
        checkForJank()
      }
    })
  }
  checkForJank()
}