I face the same problem. I just made some screenshot of the synology assistant measuring the diskstation's performance:
I am using a 207 with 2 500gb drive disks. Just to test, I have been transferring a folder containing a large avi (1.45 gb) and several small preview files. After running a couple of minutes, I canceled the transfer to make use of another protocol to make this comparrison
This transfer rate has been achieved by using wlan N connection.
As you can see, SMB and AFP are very unstable and really really slow. I am not able to stream video's to my mac via wireless. When I use windows 7 within mac OSX ( using VMWare), I am already able to cut down the expected time from 35 minutes to 8. If I use FTP however, I have achieved the highest transfer rate, the download was finished after 5-6 min.
I was about to buy new harddisks, as these are already 4 years old and getting a bit noisy, however, I don't believe they are the main cause of the slow connection (as the ftp and windows connections are reasonable). Does someone else have any idea how to speed up file transfers directly in Mac?