I'd almost certainly put this down to the server being slow. My guess is that it's bandwidth limited.

Although the site has quite a few images that could be optimised, the actual download time for such images is very slow from where I am.

For example, we can take concrete5 out of the equation and directly access a file as a test. When I do this the download speed is VERY slow. As an example, viewing this file directly:
http://webmarket-test1.com/application/files/9414/4240/7293/block_1...
took over 16 seconds to download. For a file of this size (200KB) it shouldn't take more than a second or so. It seems to trickle in and slowly appear in the browser, like it's dial-up (..shudder..)

So I don't see this as a concrete5 issue, as the webserver itself appears to have slow download speeds.