Cache images without etags and/or 2 year expiries
Images without etags and/or cache expiry dates 2 years in the future are rarely cached. I don't know if it is the lack of etags or the 2 year expiry or both that is causing the behaviour.
This means Edge requests the images every time you visit a page with such images making it much slower.
Edge does send an if modified since request if you hit refresh and gets 304s back so ironically it canbe faster to refresh a page than click the URL again because a refresh will not reload all the images.
I logged this as a bug at https://connect.microsoft.com/IE/feedback/details/1926263 but it has been rejected as a feature request so I'm logging it here.