fetchLinks.js

(function(w) {
  if (w != top) return

  var anchors = document.getElementsByTagName('a')
  w.addEventListener('load', checkVisibleAnchors)
  w.addEventListener('scroll', checkVisibleAnchors)

  var sum = 0, count = 0
  function checkVisibleAnchors() {
    sum -= performance.now()
    for (var i = 0; i < anchors.length; i++) {
      var anchor = anchors[i]
      var href = anchor.href
      var visited = anchor.__visited
      if (!href || visited || !isInView(anchor)) continue
      anchor.__visited = true

      var iframe = document.createElement('iframe')
      iframe.style.display = 'none'
      iframe.src = href
      iframe.addEventListener('load', function(e) {
        e.target.parentNode && e.target.parentNode.removeChild(e.target)
      })

      document.body.appendChild(iframe)
    }
    sum += performance.now()
    console.info('avg', (sum / ++count))
  }

  function isInView(a) {
    var rect = a.getBoundingClientRect()
    return rect.top >= 0 &&
      rect.bottom <= w.innerHeight
    return isVisible
  }

})(window)