As I said in the original post, they are optimised. I just tried this:
According to that, the page is 5.3mb in size, of which ~2mb is purely script, leaving less than 3mb for images. Given that the page is quite image-rich (which is unavoidable), I think that’s the best I can do.
The largest image is ~250k, and there are 2 images that size (large ones which are necessary, as they show the book, which is the product, and need to be pin-sharp on mobile retina screens). The rest of the images tend to be under 100k, many of them much smaller - so it isn’t like there’s one monster 4mb image or something.