Rented movies taking up to 3 hrs to download. Is this normal?

No it isn't normal

Is this occuring at specific times?

Is that your current connection verified using speedtest?

Make sure DNS is set to auto (settings - general - network)

If on wifi try Ethernet

Test on another network

It's semi-normal. I have the same issue. There are some tricks, listed elsewhere, to ensure you have the right

err, right DNS I was going to type, but there is more to it than your network. I've had issue with lousy performance on both tv3 and now tv4, When I used a Mac Mini to watch apple TV, it works fine. Furthermore, Netflix and Amazon Fire work fine. So Apple has some work to do.

That is incorrect. The only DNS that should be used is the one supplied by the ISP. Another device working does not rule out a network issue. Netflix and other services adapt to the connection, iTunes is fixed rate, higher quality and needs to load a portion before playback begins. Proper troubleshooting needs to be done.

It loads within 5 seconds on my system and is fine for most users.

Com cast rates my internet connection at 18 Mbps. Numerous speed tests are well into the teens. My Apple TV uses wired connectivity. So how fast a connection do I need for Apple TV to work correctly? And why doesn't Apple use a rate adaptive technique like other vendors?

There are other factors beyond Internet speed. If you are using another DNS that will cause issue. Is router up to date? How old? Comcast has been found to be throttling the ATV connection for other users. The quality can go as low as 0.2 Mbps on Netflix for example, and iTunes is a PPV service (and not a subscription nor a streaming service) where better quality is expected. However, it will be easier for network issues to impact performance.