It's purely down to two things:
1. The speed of your network
2. The speed of your Internet connection
I would usually suggest a user checks their sustained internet speed at speedtest.net or similar.
For near instant streaming from teh internet, AppleTV requires around:
2.5 Mbps for SD
6 Mbps for 720p HD
8 Mbps for 1080p HD
Often, but perhaps less so as internet speeds imrpove (I'n still on 4-5 Mbps ) the LAN speed will be more rate limiting in the future.
If the user is not getting adequate speed for a given resolution, they can change download option from 1080p to 720p HD in Settings for itunes store on AppleTV or in future rent in SD.
I believe there is something else. I have a 48/2 Internet connection according to speedtest(inline with what ISP promises). My appletv is connecting over cable, not wireless and movies still buffer for 1-2hrs. I also have a first gen ATV and same issue. This only started happening in the last 45 days. Previously, not an issue.