See issue739 for history.
See also this exchange between Ian and me:
http://irclog.perlgeek.de/darcs/2009-10-26#i_1644791

Looks like we need somebody who knows something about curl programming (or who'd
be happy to learn) to look into this.   Adding Dmitry because he has some
experience here.

Note that there's still that curl binding that the Galois folks made...

Note also this particular bit of msg4023 (from David): 

In particular, the problem is that darcs is taking a long time while doing the
download.  But the downloading is all done by a FFI call (to a C library, I
presume... it would depend on how darcs was compiled), and we can't do anything
while an FFI call is running unless we run with multiple threads.

Hi Alexey,

Thinking about packs fetching is making me think about improving our
progress bars when downloading content.  Would you be interested in this
ticket?  I'm provisionally assigning it to you for my tracking purposes,
but please feel free to say no!