I see the same thing between my three hubs. It's been doing this since I got it, I reported it the end of last year, here.
I have two C5s and one C7. Both C5s always download quickly, the C7 is always much slower. Other than being slower I notice no other difference.
My C7 is connected to a wireless AP, as is one of my C5s, The other C5 is connected directly to my main router.
