One thing to check... Run speedtest.net. There are occasions when the problem is my ISP narrowing the bandwidth, throttling users, etc. and other times when I'm getting 30Mbps throughput but the movie is still taking forever.
In the latter scenario, try an obscure release... If every release you try to rent takes forever, then Apple's pipe is clogged. If only a particular movie does it (and I've noticed this with popular new releases) then there may be a large number of users trying to access that file (however many times it may be mirrored it's still finite).