Gaps in Waterfall charts

Support
1

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_03_b35dd28cc1d702a3452860129b4da38c/
(Request 70-71 - ~1.2s)
http://www.webpagetest.org/result/150731_0E_aab6b13994a2dd16010a320a5888f729/
(Request 71-72 - 0.142ms)

Tsn.ca (Firefox)
http://www.webpagetest.org/result/150731_MQ_4f23b477632849c196ab76e8356d14d1/
(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?

2

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.