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 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