Thanks for the test data.
By using capital letters KBPS you mean kilobytes (kBps) per seconds and not kilobits (kbps), right?
Normally network speeds specs are given in Mbps (megabit per second), so in your case the fastest rate would equal 3.66 Mbps and the slowest 0.6 Mbps.
The biggest bottleneck here seemes to be the USB option. From wikipedia even USB 1.0 lowspeed mode should manage 1.5 Mbps. But maybe the combination of the subway/SMC EZ/WNCE2001 is the problem.
It would be interesting to know what causes this slowdown.
Have you tested the Subway USB/SMC EZ with ethernet cable direct to your windows server og via your router?
If its still very slow, test the subway for USB transfer speeds (direct to a usb storage).
Then test the SMC EZ itself on another computer (windows/mac), just to see if the amiga drivers could be the problem.
