Yes. This happens/happened to me as well. I subscribe to about 5 magazines and when I got my new iPad, restoring the back issues took forever. And yes, I needed/wanted the back issues instead of just having the current one since the issues are included to my account. Having back issues is always a good thing when reading magazines. In any case, I proceeded to download/restore the back issues for all the magazines I had. However, it didn't allow me to download more than one issue at a time. So, I just let the back issues download and went back to normal iPad usage. 15-20 minutes later, the progress bar had not moved at all for any of them. It's irritating. Even at my 25MBps speed, 600-700MB seems to take forever. Especially when you are downloading back issues and waiting for one to finish and start the download.
Settings > Store > Automatic Downloads
The automatic downloads doesn't download the magazines. I don't know if it's related to it but I receive a notification when there's a new issue. I have to go to the magazine and download it manually. Also, isn't it only applicable to Music, Apps and Books as listed?
EDIT:
Turns out new magazine issues can be downloaded in the background if turned on via the Settings in the Store.