SD movies download speed faster than HDs !

How are you obtaining the the speed information?

One thing to note is that when you download an HD movie,
you are actually downloading 2 files (sometimes 3 depending on the movie).
The down load speed reporting of iTunes can be misleading.
If you click on downloads in the side bar, you will see the multiple
files. Now, for whatever reason, it seems one file will get a "priority"
and will consume most of the bandwidth and the others get a greatly
reduced bandwidth until the "priority" completes, then bandwidth gets
reassigned. This happens if the "allow simultaneous downloads" is
selected.

However, if the download speed you were getting was from activity monitor,
then that is a different issue, and not quite sure what your problem may be
as I have personally not seen this.