Just moved to a 2820 already on 3.3.5.2 and have the very slow / intermittent problem described alesewhere. Hosts file gets round it but not sure how to do that on iPads etc!
Thinking of downgrading to 3.3.3 as suggested elsewhere - or is there another solution? Used to work fine on my simple DLink modem!

As far as I know there is no loopback issue on the latest V2820 f/w (though I might be running the latest beta and not a release, so check you have that from support if you have a NAT loopback issue)