Downloaded again yesterday and it was fast enough (several MB/s)
Assumingly the original issue was a network quirk
Well....
If you ask me, mirroring releases on GitLab would still be nice.
Of course, this needs to be decided by the team. So for now this is just my opinion.
Also, if it were to happen, the question of the work that is involved remains. Somehow the files need to get there.
=> One way would be to have a script (GitLab API) to create a "GitLab release".
Since the individual installers come from different people, the best way would be for a script to download them from sourceforge, and upload them to GitLab (uploads are part of the "project" API).
But it could also be checked with all of the people (involved in building releases), if the could directly upload and send the links (to the uploaded files on gitlab) to one maintainer, who can then create the release.
They would still need a script that takes a list of files, uploads them, and prints the links.
If all the files are uploaded, the release can probably be created via the GitLab web interface.
Though if created by a script, links to sourceforge could also be added (I would not expect anyone wants to add them one by one via the web interface).
If someone wants to help and create such scripts (depending on what the release-builders think they need), then I could go and ask the team, and the builders....
(for testing, just create your own dummy GitLab account or repo, and play with Deployment and Release)