If it's the (static) images, you should try to reduce the size of the images. There's no practicable way to do this "on the fly" on the server as this will only cost time. I think the benchmarking pagepeed plugin from Google (available for Firefox and Chrome) offers you a link to a site that optimized the image filesize. You can then replace the static images with the new versions.
You can use eg. firebug and it's network tab to determine what elements of website weight how much kb.
There is also a mod_pagespeed for apache. That can be configured to compress images, js and css and does many more optimization. However this mod should only be used if you know nothing about configuring your webserver and will need some testing as it could break some functionality of your site.
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