I have a small (Rails 3.1) app running on Heroku (with images also served from Heroku) The images load progressively (from top to bottom). If I take the same images and place them on another (Apache) server, they seemingly pop into existence once they are loaded.
So why are they loading top-to-bottom on Heroku? Is it just that they are loading really slowly (so the progressive download is obvious) or is it something to do with the way Heroku serves images?
which server are you running under heroku? Did you use a procfile and if not (and the app is using webrick) you could try thin or unicorn and see if that makes a difference and speeds things up. Example http://michaelvanrooijen.com/articles/2011/06/01-more-concurrency-on-a-single-heroku-dyno-with-the-new-celadon-cedar-stack/