Personally, I use JD w/my RG acct (macOS) but over the past couple of weeks or more, I've seen many of my D/L's peter out at about 50%. D/L speed drops from the MB/s range to the 10KB/s range and drops from there until the link dies, or I manually Disable the link, then Re-Force the download so that it catches again and hits its usual speed. But that defeats the purpose of a "download manager," doesn't it? I mean, having to keep an eye on every download and manage the downloads yourself.
After many years of using both JD & RG, I think they're both culprits. I no longer see JD as just a simple, free download manager. I don't know what the heck goes on with it "behind the scenes," but sometimes for testing I'll just d/l a RG link in my browser and I get way better speed than I get using JD. The same for FileFactory on JD, which is just atrocious. Nine times out of ten I have better success using a browser for FF d/l's than using JD.
Along with JD's overall C-minus grade from me, since RG changed hands years ago, it's been uneven in its delivery of d/l speed as well as its responsiveness to customer service inquiries.