Well, doesnt a NSLU have only 10 Mbit Ethernet???
Given that 1 MB/sec streaming performance is quite good and close to the maximum. The theoretic max. of a 10 Mbit Ethernet is 1.2 MB and thats without overhead!!!
And then theres the USB issue. I am not sure if the ports are USB2 on the slugs…
So I would not assume that more than 1 MB/sec is possible without exchanging most of the device.