Purchased movie download

Apple TV does not have storage, it streams content, the hard drive only keeps enough to prevent buffering, hence it has to download each watch.

You could stream it from the iMac from iTunes, this could save download capacity and be quicker depending on your set up. I stream direct which suits me but know people who airplay movies fron the iTunes well. It does not affect the ate it can just be a better connection but it all depends on network capability.