I don't have much data myself but when I was doing scraping some time ago I had thousands of examples where f.ex. the full-res image was something like 1.7MB and the blurred image was in the range of 70KB - 200KB, so more or less 7% - 11% of the original. And I might be lying here (it's been a while) but I believe at least 80% of the blurred images were 80KB or less.

Technically yes you could make some savings but since images were transferred over an HTTP-1.1 Keep-Alive connection, I don't feel it was such a waste.

Would love to get more data if you have it, it's just that from the limited work I did in the area it did not feel very worth of only downloading the high-res image and do the blur yourself... especially in scenarios when you just need the blurred image + dimensions first, in order to prevent the constant annoying visual reflow as images are downloaded -- something _many_ websites suffer from even today.