Chevereto already does a fast load by using medium sized images by default in listings and in /image/<id>. Big resolution image are loaded via JS so the user experiences a fast load indeed.
At Chevereto we do resampling, which generates better looking results. To be honest I haven't compared pure resizing vs scaling, I will run a test later on.
By the way, Pagespeed or any bot like that will always tell you that the image could be smaller without affecting quality and that is the most ugly lie that I've read in recent time. Is just not true, even using the most aggresive algorihtms that thing will always tell you that a jpg could be smaller.