• Distance from chip determines minimum latency
• Speed of light is a constant
• Impact of bus frequencies
• IDE (66,100,133 Hz)
• PCI, PCI-X (66,100,133 Hz)
• PCI Express (1Ghz to 12Ghz)
• Impact of bus bandwidths
• PCI (32/64 bit/cycle, 133MB/s)
• PCI Express (x16 8.0GB/s)

Here's a link from Intel showing a machine architecture with signal bandwidths: Intel diagram