I have a DSL-320L connected to a network with both Windows and Mac machines present.  When I access through the Windows station, the performance is exceptional.  When connecting through the MAC, it's dismal.  I have more experience with PC's than I do with MAC.  I'd appraise the issue to be a behaviour where the MAC needs to cache a version of the file allocation table before any data can be read.  The primary reason for the DNS-320L is to act as a shared photo server. So, because the files are large by nature, getting to a point where the file can be viewed and dealt with is a time consuming burden.
On the PC, it's fast like a local drive.