I think Eddie is not so far away from the truth.
The DIR series is designed to be a router.
An additional print server needs additional memory to handle print jobs and causes processor load.
NAS functionality needs a lot of processor resources -> just see the performance of a 500MHz NAS as the DNS-323. DIR-655 has a 275MHz processor only. What performance do you expect ?
What will happen with your bit torrent when streaming a video from a device connected to the DIR or printing to a connected color laser ? Don't be disappointed if the performance will decrease.
Also the shareport is more versatile. Most printservers can't handle multi function devices or standalone scanners.
Shareport can...
And most of the work has to be done by the computers connected.
As the DIR-655 is a SOHO router, I think it is no real disadvantage that only one can use the connected device at a time.
My suggestion to D-Link would be to enhance compatibility of the shareport and optimize performance and the handling of the devices.
And please don't forget the MAC users on the long run

(Though they should have known it before ... )