For an explanation of of why performance will drop over time see this article: http://www.anandtech.com/storage/showdoc.aspx?i=3531&p=4
However this problem shouldn't cause the drives read speed to drop to the levels you mentioned, and I would go through ChileanLlama's suggestions first.
However this problem shouldn't cause the drives read speed to drop to the levels you mentioned, and I would go through ChileanLlama's suggestions first.