WebPageTest :: How can Start Render be after the load event?

Question

What can cause the start render to be delayed even after the load event has fired?

Look at this webpagetest result

Answer 1

If you check the page you tested without javascript (F1 > Disable Javascript in Chrome DevTools), you can see that most images are lazy-loaded, which mean they are loaded via a JS script checking if the images are in the viewport. As you can see, without JS the load time line is well after all assets loaded.

Also for other assets, like JS, you can add an async or defer attribute to de-synchronize your scripts loads.

Finally, some scripts can be even deliberately loaded after the load event using an attachment on the window load event to write <script> tags in the body when it's triggered as loadJS do for example.

You should read that by the way to better understand how rendering and events timing work.

Answer 2

This is most probably a bug. For the moment, the iPhone instance on WebPageTest is quite recent and the author is still improving it:

The iOS support is just becoming stable though it's still under VERY active development so don't be surprised if there are a few rough edges (and a lot more limitations than the android testing). - Patrick Meenan, 09-09-2015

The android tests are much more reliable.