John_H, I disagree your opinion.
PCI traffic and performance heavily depends on chipset system. We observed that board with same PCI designs may have dramatic speed changes on different chipsets. Chipsets from some manufactures have a better performance, some have bad performance. Usually the latest version of chipsets will have a better performance over older versions for the same manufacture.
It is first time to hear that in a new chipset system, there is a 20% speed decrease. It means to me that most likely the new chipset has a different schedules than before that may put PCI transactions 1 level of order of delay. For example, for older version of chipset, the schedule algorithm in the chipset selects next request to go based on their arrival time from a incoming queue; for newer version of chipset, the schedule algorithm in the chipset may change to response PCI request after 4 CPU requests are answered. If so, it would appear and report that CPU gets faster performance while sacrificing PCI environment that nobody pays attention to.
I don't know why 20% speed reduction really is. That must be confirmed by PCI bus analyzer. But the reason John_H indicated is the least likely real reason.
You can imagine with faster DDR/DDR II system, possible two channels of them and much higher working frequency, a 20% performance decrease cannot be blamed on the board side.
Weng