When the Mac Client Window is
completely hidden the client is unable to keep it's downloading rate.
The window can be hidden by another application window or if you use multple desktop.
Partially visible client is not affected by that strange behavior.
You can visualize such a behavior with a tool like Little Snitch Network Monitor :
This may explain why some people complain the client is so slow to download the game or an update, because they don't keep the window in front.
Also, I guess this is a side effect :
While downloading the client prevent the Mac to go to Screen Saver or Energy Saver (screen off) ; If you pause the download it's ok.
This is annoying if you get a low bandwidth and you want to let your iMac turn on all the night ...
Another really small bug : the client keep showing the last bandwidth rate even if it is paused.