This page contains a <SCRIPT SRC="..."> tag in the middle of the page that takes 10 seconds to load. This illustrates two problems.

PROBLEM 1: Everything below the script won't render until the script is loaded.

The <SCRIPT SRC="..."> tag is here.

PROBLEM 2: All components below the script don't start downloading until the script is done.

The script blocks these images from being downloaded, so they're not available until much later during rendering.
     

It's even worse if scripts are in the document HEAD. Look at Example 2 - Scripts Top vs. Bottom to see a comparison of scripts at the top vs. the bottom of the page.