people forget that the first ipod was firewire only and firewire was a very apple thing at the time. it wasn't until they gave in and changed it to usb (and released iTunes for windows) that it really set the world on fire.
USB 1.1 was 12mbits/s -- that is 1.5MB/s. Firewire was 400mbit/s.
Accounting only for bus speed, filling up a 5GB iPod would have taken 54mins[0] for USB 1.1 vs 1.6mins[1] for Firewire 400.
[0] 1000MiB/1024MB * 5000MB = 4,883MB / 1.5MB/s = 3255s
[1] 4,883MB / (400mbit/8) = 98s
Apple even made dual Firewire/USB cables to allow the (Firewire) charging brick to be used while syncing.