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WebPagetest Forums - Gaps in Waterfall charts

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I'm running into some issues where looking at the waterfall of some sites, we see gaps where no traffic appears to be sent. Some examples are below:

Bestbuy.ca (Firefox)
http://www.webpagetest.org/result/150731...29b4da38c/
(Request 70-71 - ~1.2s)
http://www.webpagetest.org/result/150731...a5888f729/
(Request 71-72 - 0.142ms)

Tsn.ca (Firefox)
http://www.webpagetest.org/result/150731...8356d14d1/
(Request 104-105 - ~700ms)


Depending on the run, the size of the gap fluctuates. The CPU does not seem to be a problem and from the tcpdump, I can't see anything network related that might be causing this. Is there anyway to determine what is causing this and possibly remove it from happening?
Can you reproduce it on any browser other than Firefox (preferably Chrome)? If you can see the gaps in Chrome then you can capture a timeline or a trace and we can see what is going on. It could be an expensive layout operation or something like that and CPU can still be a constraint even if it doesn't show as 100%. The thinkpads are dual-core for example so even the main browser thread being 100% busy would only show as 50% overall CPU.
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