Mac OS X Server 10.6.4 Snow Leopard Finder slowness using AFP

There are 4 Macs in my house. I use two of them for work and one as a server and media center for the occasional video or TV show (the last Mac is Pam's). All of my media is stored on a NAS box that supports just about every protocol out there, and since every computer in the house is a Mac, I use AFP to connect to it. A few weeks ago, the Mac Mini that acts as the media center and server started slowing down. All Finder actions were sluggish, and opening folders started to take a few seconds (even on local disk). Opening folders and files on the NAS box (connected via AFP) suffered the most, and the Finder would sometimes take 30 seconds or more to return. Sometimes, files in networked folders would never show up, leaving folder windows empty (and spinning). Obviously, streaming video files from the network became impossible to do. Meanwhile, access to the networked files from all of the other Macs in my house was as snappy as ever. In fact, I started copying files from the NAS to the Mac Mini by mounting both volumes from a third machine and copying between them (using AFP) -- terrible, I know.

On a whim, I unmounted all networked volumes on the Mac Mini and re-connected using SMB. Problem solved -- everything is now as fast as it used to be. Why is it that 3 machines (Mac Pro, Macbook Pro, iMac; all running Mac OS X 10.6.4) connect just fine to my NAS box using AFP, but one (Mac Mini Server running Mac OS X 10.6.4 Server) must be connected via Samba for suitable performance?

I'll suspect that I'll never know.