I queried the average onLoad time from 2010-2015, and plot it below.
Why there is a clear drop of PLT from 2012-2013? Adoption of Http2.0?
According to Wikipedia, that would be when companies other than Google started using SPDY. HTTP/2 would come later.
I believe the differences are due to change in the test environment: http://httparchive.org/about.php#testchanges. @pmeenan and @stevesoudersorg would know better though.
Thanks for the pointer. This makes a lot of sense.
Even after 2013 with the same test setup, the PLT keeps increasing. I check the number of connections and domains almost most none changed. I am wondering the major reason then, page size increase?
As they say: it's complicated. As a start though, check the correlations graphs on httparchive.org.