I have a really weird networking issue on my Mac Pro (Early 2009). I can’t get higher network speeds than about 2MBit/s. It doesn’t matter if this is over AirPort or one of the Ethernet ports. An iMac and a Mac mini in the same network with almost the same configuration get about 25-30 MBit/s. I’ve read a couple of things about this on the official Apple forums, but there is no helpful information. Anyone else with Mac Pro network speed issues and who knows how to solve them?
How are you measuring performance?
What numbers do you get with IPerf? IPerf is a simple command-line tool for reliably measuring network performance. It does TCP-based performance by default, and it's written to take full advantage of the speed of TCP, unlike some web server / web browser and file server / file service client implementations that get bogged down in their own higher-layer issues.
Why are you capitalizing the B in bits? I only ask because many fellow geeks conventionally reserve a capital B for Bytes, and a lowercase b for bits, to help avoid being off by a factor of 8 when talking about network speeds vs. file I/O speeds. So it makes me wonder if you confused Bytes vs. bits in the output of whatever performance tool you were running.
While 25-30 megabits per second is awesome for 802.11g, it's too slow for 802.11n and way too slow for even 100 megabit per second 100BASE-TX Fast Ethernet, not to mention 1000 megabit per second 1000BASE-T Gigabit Ethernet. Are you sure that your iMac and your Mac mini only get 25-30 megabits per second over Ethernet?
Even if you meant Bytes, 25-30 MebiBytes per second would be about 210-252 megabits per second, which would still be way too slow for GigE.
I'd like to see you hook your Mac Pro directly to your iMac or your Mac mini (whichever is more recent), no switch in between, and you don't even need a crossover cable because Macs have always had auto-crossover (auto MDI-X) Ethernet ports for over a decade now. Then I'd like you to run
Run IPerf in server mode on one machine:
and in client mode on the other machine:
...where that 169 address is the self-assigned IPv4 link-local address of the IPerf server machine.
Oh hey, one more thought, make sure you're not manually overriding the speed/duplex/flow-control/MTU autonegotiation on the Ethernet interface of you Mac Pro. It's under System Preferences > Network > Ethernet > Advanced > Ethernet.
Why not call Apple support on this? They are extremely helpful and could probably troubleshoot and find an answer for you in no time. I think if you're still with in the first year of ownership it can be covered as warranty issue.