Billy:

EMS/Jomar sells a small box that will adjust servo speed. Intended to make the landing gear look more realistic, but it will work on your cargo door just as well.

It is the first item on [link=http://www.emsjomar.com/SearchResult.aspx?CategoryID=6]this[/link] page, it's noted as a gear door sequencer but it also has adjustable rate up to 10 seconds.

Bill.