かきすて　しかし、気がつけばスクロール同期＋キュー管理へ。

chikoque.js

/**
 * Chikoque
 *
 * @author ahomu
 */
(function(win, doc, _) {

  'use strict';

  var defaults = {
    dataAttribute: 'data-lazy',
    parallelLimit: 5
  };

  var Chikoque = function(options) {
    options || (options = {});
    this.dataAttr   = options.dataAttribute || defaults.dataAttribute;
    this.limit      = options.parallelLimit || defaults.parallelLimit;

    window.addEventListener('scroll', _.throttle(_.bind(this.subscribe, this), 200), false);
    this.subscribe();
  };

  function _createQueue(img, dataAttr) {
    return function(done) {
      img.onload = img.onerror = done;
      img.setAttribute('src', img.getAttribute(dataAttr));
      img.removeAttribute(dataAttr);
    };
  }

  Chikoque.prototype.subscribe = function() {
    var clientHeight = window.innerHeight || document.documentElement.clientHeight,
        pageYOffset = window.pageYOffset,
        from = pageYOffset - 200,
        to = pageYOffset + clientHeight + 200,
        imgYOffset;

    this.idleLazies = [].slice.call(doc.querySelectorAll('img['+this.dataAttr+']'));
    this.imgQueues  = [];

    var i = 0, img;
    while ((img = this.idleLazies[i++])) {
      imgYOffset = pageYOffset + img.getBoundingClientRect().top;
      if (imgYOffset > from && imgYOffset < to) {
        this.imgQueues.push(_createQueue(img, this.dataAttr));
      }
    }

    if (this.imgQueues.length) {
      this.kick();
    }
  };

  Chikoque.prototype.kick = function() {
    if (!this.running) {
      this.run();
    }
  };

  Chikoque.prototype.run = function() {
    var that    = this,
        limit   = this.limit,
        running = 0;

    this.running = true;

    function onloadHandler() {
      running--;
      if (running < limit) {
        nextQueue();
      }
    }
    function nextQueue() {
      var queue;
      while ((queue = that.imgQueues.shift())) {
        running++;
        queue(onloadHandler);
        if (running >= limit) {
          break;
        }
      }
      // if not exists remaining queues
      if (!queue) {
        that.running = false;
      }
    }

    // start
    nextQueue();
  };

  if (typeof define === 'function' && define.amd) {
    define(function() {
      return Chikoque;
    });
  } else {
    win.Chikoque = Chikoque;
  }

})(window, document, _);