title: Find layer rendering time in mapping API's
body: <p>I am trying to compare performance of mapping API's- Google Maps, OpenLayers, Leaflet and ArcGis API and I would like to compare time of vector layers rendering in each of them. 
  I want time, when all vector features are already on the screen. Tried     <code>performance.now();</code>, but this gives wrong time.
  And as I am trying to compare API's I'd like this method to get time of rendering to be the same for every map. Is is possible to do 
  with some general method??</p>
  
  <p>For example in OL, vector layer is defined like this, and I want time when all features are on the screen.</p>
  
  <pre><code>var vectorSource = new ol.source.ServerVector({
          format: new ol.format.GeoJSON(),
          loader: function (extent, resolution, projection) {
              var url = '--------';
              $.ajax({
                  url: url,
                  dataType: 'jsonp'
              });
          },
          strategy: ol.loadingstrategy.all
      });
      var vectorLayer = new ol.layer.Vector({
          source: vectorSource
      });
      var loadFeatures = function (response) {
          vectorSource.addFeatures(vectorSource.readFeatures(response));
      };
  </code></pre>