The "download all submissions" feature often takes a long time to process all the submissions into a zip file, and there is no indication that anything is happening. It would be very useful if this could show a progress bar while the zip file was being created.
As well as being a good improvement to the user experience, a specific issue this would solve is where Moodle is run in a load balanced environment and the zip file takes longer to create than the timeout for the load balancer to receive a response from an upstream server. In our university, we're seeing this happening with a 5 minute load balancer timeout on an assignment with 130-odd students who have submitted a few (sometimes large) files each.
The same issue used to happen with backups, and this has been pretty much eradicated by the use of the progress bar, as the load balancer starts getting a response fairly quickly even though the whole request still takes longer than the timeout to process.
I realise this may be awkward to implement as the page is currently serving a zip file directly, so you'd need to move to having the page create a zip file then redirect to another URL to actually serve the zip file, but I think it would be worthwhile.