Identify blocking or gap causing request

I’m running tests against a site with a lot of third party requests.
I see that often the doc ready is very slow (65 secs)
I’m trying to identify which request is causing this slowness, but I don’t see any blocking requests higher up in the waterfall.

I have read the blocking might be caused by javascript execution. How do I identify which request is running javascript causing the blockage?

I’ve add a partial screen shot of the waterfall view, I can add the rest if it is useful.


Performance tab of Chrome developer tools is what I’d use to debug this further but from this waterfall WebPageTest Test - WebPageTest Details it looks like

  1. looks like a candidate for an initial investigation - notice how it’s JS execution extends to onload

  2. I’d be tempted to take a close look at Optimizely too and maybe consider moving it so it loads sooner