I just synchronized my items to my phone and I hit Select All and then Download. My phone had a small heart attack and I'm pretty sure I just DOS'd a few podcast servers. The error log shows a few that say No resources (or something like that). Meaning I'm guessing I hit their server too much and they rate limited me? Putting downloads in a queue would be a good idea and limit to x at a time. Or 1 download per resolved IP.
Mmm, it does indeed start all downloads simultaneously. When I added this bit, I looked into the qt5 documentation, and remember that it said that it would automatically limit the amount of simultaneous connections. I'm either remembering it wrong, or something changed in the meantime. Anyway, the history doesn't matter anymore. :) This should be fixed.