My guess is that that is a break between the and the and it’s all of the stuff in the head that is causing the break. That’s a pretty huge time to load a few of those css and javascript files, are they really large and uncompressed?
It’s also possible that there is something on the back-end that is delaying the page after the head (maybe it gets flushed once at the end of the head and then does a lot of expensive work before flushing the rest of the page).