Upload Speed Grinds to a Halt Thread

When trying to upload a file (2.06 GB .7z archive) to my mod, the upload speed starts out at a reasonable pace but then within a minute or two invariably drops down to a fraction of a kb/s. I was able to successfully upload a different file (only ~20 MB) earlier on the same day, but after repeated attempts and waiting another day to see if it resolved itself, I still have the same problem.
So, is this a known issue? Is there anything I can try to fix it? I don't have access to an FTP server for a from-URL upload. Sorry if this is a duplicate, I couldn't find any threads with this specific case.

Edit: I've now tried it from a completely different location, with a much faster connection, and the exact same thing happens.

Edited by: TKGP