[W]e're trying to limit our search to things we
could get, rather
 than make, 
Ah!  Okay.
  Construction is likely to be bit serial, and the
target is 500k -
 1000k bits per second through the loop, maybe an instruction rate of
 10% of that. 
That's higher than I'd expected, especially in view of your "20kHz is
about the max available frequency" note.
  I will say some of the replies on this group are
realistic -- when I
 mentioned this at work one colleague (Windows Programmer) asked if it
 would run a web server.   Sigh. 
Heehee!  Sure it will - just verrrrrrrry slowly. :)  (Hey, someone has
a webserver on a C=64, I think, and that one actually borders on
usable.)
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