I should think its down to the page size; according to yslow:
This page has 17 external Javascript scripts. Try combining them into one.
This page has 8 external stylesheets. Try combining them into one.
There are 10 images that are scaled down - I can understand scaling them down for mobile but why on a full screen?
I had a similar problem a few years ago and it was worse on different browsers and part of the problem was different browsers load content in different ways. One browser loaded the images as they came and another loaded them after the rest of the content.
I got around this by only loading the content with an include and left the main part of the page the same. The URL contained the name of the content to include but I do not think this method would work in your case.