This issue have been brought up by many users, yes sometimes images taking ages to load. I'm not sure but I guess simply enabling caching on proxy server should solve the issue.
Yes first time it will take time, but later when user reading the post it will load pretty fast. (it's a very naive solution probably it's already enabled and issue is something else)
However it also depends where you are upload your images, I see
imgur.com is the best in terms of speed comparing to
prnt.sc. I will suggest use imgur and try not to use original version. Imgur have option to show images in different seizes Small Square, Big Square, Small Thumbnail etc
User the appropriate size for example most case Large Thumbnail and Medium Thumbnail works for me and loads pretty fast as compared to Original
Also you can use alt and put message like "Image loading", so the reader will wait for the image to appear.
[img alt="Image loading"]https://i.imgur.com/qCrG1LW.png[/img]
https://i.imgur.com/qCrG1LW.png