Downloading is not only slow but on iOS devices you must also ensure that the screen does not turn off and the ARC app does not go to the background, else it seems to stop downloading because of background battery saving.
On Android it’s possible to turn off battery optimization on a per-app basis and then it continues downloading in the background.
It’s slow to download on both platforms though. Don’t ask me what’s going on there, both valid complaints have been there since day one. Many threads about this, see the search option 
E.g.,