well, i made some test.
i tested both HFS (1.6 and 1.5) and Filezilla (FTP).
sadly, the CPU is consumed by Windows itself.
you can see it by yourself using the task manager.
in facts, during my tests, the FTP server also brought the CPU to 90% and always an half of this was the System task.
i also tested using the ttcp util (by microsoft) to transfer ~100MB.
CPU usage reached 100%.
I'm not aware of a method to improve to solve this problem. Are you?
Moreover... Testing i noticed that Filezilla used to reach 6MB/s while HFS only 5MB/s.
This means speed can be improved, but this may require to change tcp library. just don't know.