What can cause the start render to be delayed even after the load event has fired?
Look at this webpagetest result
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.