Hi
I assume you are on wireless for all devices and that you have a router?
The computer should not really come into the equation when using Netflix.
Both the iPad and the AppleTV should access the internet directly to get the Netflix stream. This should work even if the computer is off unless you have configured the computer to share an internet connection over wifi.
There are many reasons this could be problematic:
1 - Inadequate internet bandwidth
2 - Inadequate wifi bandwidth/signal issues on your LAN (local network at home)
3 - A problem with Netflix
4 - A problem with the AppleTV
In theory if all devices are on wireless I would expect mirroring the iPad (assuming Netflix app on that) to the AppleTV to be more stressful on the local network than just trying to stream Netflix on the AppleTV direct using the silver remote. So if mirroring works ok your LAN is probably up to the job.
A few questions:
Have you set up Home Sharing on the computer? If so, can you stream content from iTunes on the computer to the AppleTV easily without issue?
Have you tried renting from the iTunes Store directly on AppleTV - if so how long does it take to be able to watch something ? Instant, minutes or hours - if it takes a long time to rent I'd be suspicious that your internet connection was slow.
You'd wonder then how iPad works ok - I've a funny feeling that iPad Netflix always defaults to a low quality stream whereas AppleTV woudl try for a better connection.
In your Netflix account on the internet is there an option to alter your preferred quality setting? Netflix has only been with us in UK for a few months but my account has such a setting with 3 quality options, higher quality requiring better internet speed.
You can test your internet speed on teh computer at speedtest.net - might be worth seeing what this gives you.
I would also check the terms of the contract with your internet provider - some of them 'throttle back' connections at certain times of the day or after a certain quota of usage.
AC