Anybody tried compiling setserial and gotten it to work?

I have been chasing a serial port latency problem and tried building setserial from sources.  Aside from the fact that autoconf seems to go into infinite recursion and needs to have parts amputated.... when I finally got it built it tells me this:

chumby-:/mnt/storage/setserial-2.17 # ./setserial /dev/ttyUSB0 low_latency
Cannot get serial info: Invalid argument

So there is something about /dev/ttyUSB0 that it doesn't like.  The device presents itself as a cdc-acm device.  I'm still running the old and crufty cdc-acm driver that comes on the CHB.  Anybody have any clues about setserial?  I'm not very familiar with the program.

-edit- I accidently the enter key

Also... what kind of latency should I expect to see for serial port service when I have a usb-serial device set for cbreak mode?  Right now it is outrageously high -- if I can't fix it, the CHB is going to become a frisbee. (By outrageous, I mean the Chumby takes 150 mSec to do I/O that takes 3 mSec on a Macbook -- this is pretty much *all* I/O time in the Chumby usb serial driver stack.)