My guess is you have a relatively fast connection. The WebPageTest test you ran was on a 5Mbps connection which is towards the lower end but still a very common actual speed.
There are upwards of 6MB of images including that tiny PCNA logo on the top left which is a whopping 1.5MB.
Not considering anything else, the 6MB of images alone takes ~10 seconds to transfer on a 5Mbps connection under perfect conditions (no time in round trips or slow start).
It looks like even a simple recompression of just the JPEGs with quality level 85 would cut it down to 1.7MB (though even spending just a few minutes on each image should get a lot better than that).