I just copied a bunch of files off of a USB Zip drive.  It took
1051.35 seconds (17.5 minutes) to copy about 92MB.  I know USB isn't
terribly fast, but a Mac is able to copy an entire Zip disk (over USB)
in about 2.5 minutes.  (The particulars of the NetBSD system in
question:  NetBSD 1.5, 350MHz Pentium ][, 64MB RAM, pretty much idle
during the copy other than handling PPP routing.)

Is there a particular reason why it was this slow, or is this a can of
worms waiting to be opened?

While I'm complaining here, I observed a very curious bit of
misbehavior:  the fictitious disklabel crafted for a Zip cartridge
isn't consistent.  For one of two Zip cartridges, I got partitions

d: 196608 0   unused
h: 196576 32  MSDOS

but the other came up with

d: 196608 0   unused
e: 194528 32  MSDOS

What determines which partition it uses, and why are the sizes
different?  The two cartridges were formatted on the same (Windows)
system, I think.  (Considering that the contents of these two
cartridges are the -current sources, I suppose I ought to just go look
up the answer myself...)