Thanks to this commit
https://github.com/discourse/discourse/commit/eb428ef54dcc1d0d74f5d4a0d3fd12f1122dab65
Uploads to the forum now process much quicker than they used to.
There used to be a bit of a delay (in some cases as much as 30 seconds) until images that were uploaded would be processed and correctly displayed in a post. They should be handled much faster now (image size will still cause a small delay as the server processes it - so a 10MB file will take just a moment, but a small image should be instantaneous).