Right now, win32 performance with libevent is about as bad as you can get. This is a shame, since win32 has a lovely IOCP system that matches very will with libevent's bufferevents abstraction.
Christian King, a student working for Tor via the EFF for this past Google Summer of Code, wrote the beginnings of an implementation for bufferevents based on IOCP. We should integrate it, or something like it.
It probably needs more work; you can see it in the Tor svn repository at
The code is based on libevent-1.3b.