Aside from "Why A3 doesn't support HTTP-download? Half-Life came with this feature back in 1998." and "Why downloading MP mission starts slow and speeds up over time?" this is some actual bug report:
The "receiving data" progress bar gets stuck rather early (for a 45MB mission (quite a lot of custom content) around the 3MB mark) and the GUI "hungs up" while the game in fact is still downloading the mission from the server, there's just no graphical representation of it.
The "issue": As the GUI "hangs" when clicking a few times the "application stopped responding" pops up until the mission is downloaded and loaded - then the game gets back to normal behaviour.
Yes, I did used the search function with both "progress bar" and "receiving data" - but couldn't find any existing report - so apologies if this is a duplicate to some other report - I wasn't able to find it.
As just a side note, not actual part of the report: It always bothered me why A3 doesn't support mission loading via HTTP(S) - even Half-Life1 supports it (although only HTTP - HTTPS only came with rather recent Source updates) - and this game is back from late 1998. Also: Although I'm aware of Nagle's algorithm this only affects TCP - but as far as I understand the mission is loaded over UDP game port - so it doesn't make any sense the first couple MB takes forever cause it loads with only a few kb/s - and when you pass the 100MB mark you already load with over 1MB/s+ - WHY? Aside from "intended throttling" to me there's no real explanation. Either go full speed from the start or throttle it to a fixed speed.
And as A3 is closing in its 10th anniversary it's maybe too late for this engine (although I guess A3 is still played even when the next major game comes to stores - so even a late addition would be beneficial) this should considered to be an option in the new engine / netcode from beginning.