If you are using 802.11n hardware (like any recent Apple product) along with your older PowerBook (which I assume has it's original AirPort card installed) which is 802.11g this can slow down the 802.11n devices.
Put simply, older WiFi hardware can slow down the network, therefore then the newer devices.
Thanks for your reply!
My slowness just started being noticed. I upgraded to 10.5.8 early in December.
I assume it's the original AP card. From system profiler:
AirPort Extreme (0x14E4, 0x4318)
Can that be upgraded to an "n" model that will still work with PPC processor under OS 10.5.8?
Regards from snowy VT.