My site's first web test has abnormal time gap.

[shren]

Hi!

I have tested my site.
Here's are my test result.
http://www.webpagetest.org/result/150315...1/details/

It has really long gap before #19 request, so total loading time is above 20s.
but if I test again immediately, it's loading time is much smaller.

If I test again much later, first test has a huge time gap always, only first.

I don't know why these time gap occurred.