Sometimes the “DOM Content Loaded” area of the waterfall is skinny and sometimes it’s fat. The skinny version always seems to show much better scores on page speed index and start render.
Skinny - 1085 speed index
http://www.webpagetest.org/result/170110_FT_6NR/
Fat - 1487 speed index
http://www.webpagetest.org/result/170110_NC_5EH/
What makes DOM content loading shorter or faster on the same URL?