2 Intel Macs; a brand-new MBP and a 1-year-old iMac. Both have good quality wired network connection to an Airport Extreme. I regularly copy a 25Gb+ file between them. The disks have plenty of space and the iMac was defragmented a little while ago too.
Most of the time it takes around 9-10 minutes to copy the file. Sometimes it takes 40+ minutes. I have no idea why. I thought that sometimes the MBP was perhaps transferring the file over the airport wireless connection rather than the wired one, but I've disabled the wireless connection on the laptop and it's made no difference.
I'm fairly sure it's nothing to do with the disks, because most days it transfers quickly and then suddenly one day it crawls along. The only other box on the network is a Dell server running Windows 2k3. None of the machines are under load when the transfer happens and there's nothing else of any significance happening on the network, as far as I can tell.
Where would I start trying to figure out what is causing the delays? My understanding is that the network should all be running at 1Gbit - how would I go about checking this out?