Q: Why is my Apple TV so slow downloading from iTunes when other sites such as HBOGo or Netflix or showtime start playing immediately.  Apple needs to fix this!

Those services adapt to the connection whereas iTunes is fixed rate, higher quality, and needs to load a portion before playback beings.

You need to troubleshoot

What is your current connection via speedtest

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

If on wifi try Ethernet

Test on another network

I'm seeing the same issue in the evenings.  With the latest AppleTV, I installed SpeedTest.net, and I'm seeing around 8-12Mbs download speed.  Plenty.  Other apps can download content quickly, but iTunes content on AppleTV downloads very, very slowly in the evenings.  The movie trailers app can barely start playing before having to buffer.  TV shows or movies download in chunks.  I've tried all the suggested fixed of alternate DNS settings, rebooting, resetting, moving the AppleTV unit, nothing helps.  The unit has a strong, direct signal to my Airport Extreme router, it has a good internet connection.  The issues seems to be with Apple being able to feed the TV and movie content during prime viewing ours.  Are others seeing the same?

As already stated those other services adapt to the connection, itunes is actually loading the file at a fixed rate and will start once enough has buffered. Speed is only one factor (though 8mbps is the minimum required for 1080p content), DNS needs to remain on auto. The signal strength shown on the device will not account for any issues. You need a network report.

If it's occurring during specific hours then that would likely be your ISP throttling, it has nothing to do with Apple. It's working fine on my system and for the majority.

Sadly, this has nothing to do with streaming versus fixed rate download.  A weak signal strength on the AppleTV could account for issues talking with the router, but that is not the case here.  There is definitely no ISP throttling going on.  I know the ISP owners, and they are leaving the pipe wide open, no blocked ports.  I can download other content just fine, lots of bandwidth on the incoming internet network.  The issue is with downloading a movie or TV show on AppleTV during the evening hours.  I can run the speed test on AppleTV itself, and see 8-12Mbs.  A regular 1h TV episode at 720p then takes many hours to download.  If the 4Gb movie takes 4 hours to download, the receiving rate is more like 0.25Mbs, a fraction of the available bandwidth.  This has been going on for quite some time.  As we now have faster internet speed over the last 6 months, the movie download issues didn't vanish.  There seems to be lots of posts on forums asking the same question, and I'm not seeing any answers to this.

Actually it does, those other services can adapt (Netflix for example cam go as low as 0.2mbps) where itunes needs a consistent connection and won't start until a portion has loaded. How exactly have you verified your network? Have you tested on another network or taken the unit to Apple? There are always going to be posts here as this is a support forum, however it is a very small group of users and when troubleshooting is done the issues are resolved, and the vast majority of users aren't seeing the issue.