Purely in terms of Web page size ( and not loading speed ), images is the biggest problem. We have been stuck with Jpeg for far too long. I really wanted Bpg to take off. It should at least save 30 - 50% off images size, which is now the biggest weight in web pages.

Then there is Web Fonts, along with JS Library. Most of them should be cached already.

So really, we need to cut the images size down. That's it!