Well spotted Francky; I assume the plugin was just supposed to be used in development and disabled when the site went live.
•The source code of the home page is ending at line 700 with the </html>.
•After that, there is a comment <!-- Performance optimized by W3 Total Cache ... -->
•I think they didn't do it very good: these comment lines are going to ... line 4247.
•For the visitor these comment-lines don't have any meaning: the are just in the code, and don't do anything.
•The page-size with comments is: 388kB.
•Without the comments the page-size is only 56kB.
•That means: this page is about 7 times faster downloading if you remove the unnecessary comments!