I can't answer the tech reason why the 343 is so slow in Raid 5 but I have similar issues with all of my 343's. I think it's related to the speed of the 343's board not your PC.
As we speak, I'm moving over 300,000 image files (50gb total) off the 343 to my Windows Server and it will end up taking about 3-4 days. That is over 100 times slower than moving them between 2 NTFS Windows computers.
Hopefully in the future, the 343's will be capable of NTFS, I'm getting weary of the EXT 2-3 file system after using it for 6 months. For all you Linux lovers, sorry nothing personal.
I only have the 343's because I love my D-Link equipment. I have all D-Link routers, 3 wireless cameras (D-ViewCam Beta version 3 is awesome), NAS devices (I have 4 now) and multiple gigabit switches.