When I first installed the ATV2 I only had 3mbps. Netflix worked but because it provides a signal based on your speed the resolution was just OK. iTunes movies in HD took hours and I had to call up and increase my speed. Now at about 12 or 13 Mbps I get really nice, instant Netflix ( with only the odd glitch) and the iTunes content can be watched in seconds. This is much better than I ever got with the original ATVs obviously. With those it would often take us close to two hours to watch a rental. I only wished I had I creased the speed years ago, although I don't think it was available too long ago.
So, if you can't increase your speed you will have to have patience. There is no way for you to get better service?
As was noted above, that should be OK for standard definition. However, for HD content i'd recommend 5mbps at the very least. Otherwise i'd disable the high def rentals and just watch SD if you can't upgrade.