How slow is slow? Usually servers have protection against slow-post attacks and the like (posting or requesting things really slowly to keep hold of threads and thus cripple the server). I’m not sure how slow it would have to be, but I’d imagine pretty darn slow.
I’m pretty sure if you’re going to use a download manager, which works a bit like torrents (different workers downloading different parts using unique connections) you should limit the connections to something low, like 3. These days - especially big companies - frown upon that kind of activity. It’s kind of abuse in many ways.