When I was using Downcast on iOS refreshing all feeds took ages and a lot of traffic (some feeds with a lot of episodes can be more than 500 kB, that adds up when you're subscribed to 50 feeds) and since the refresh wasn't even threaded all it took was a single slow server to block the whole refresh.

Now in Overcast (which does server-side crawling) the refresh is instant and I can even get immediate notifications about new episodes.