But there are many reasons for this. From folk that install an antivirus on their Apple Mac OS X to a bum cable, drive or board.
The firewire port speed test should be almost ignored as the firewire speed even if it was 800 would only get you 80'ish megabytes per second with everything working in top form.
With my PC computer I'm getting 85-95 mb/s read & write speeds on internal SATA drives that are 80% full.
Working on a 2yr old MacPro Tower (12gb ram, Xeon 2.8 quad core) I am reading only 65-68 mb/s on drives less than 50% full. Same type of Seagate barracuda 2TB drives in each computer. Is there a setting or bottleneck issue? Also plugged in a Graid 4TB drive (Raid 0) using firewire ports and speed tests with it are only about 10mb/s faster.