Many people have complained about File Sharing hogging processor cycles, and have reported better system behavior after disabling File Sharing.
I came across the UNIX command nice, which says you can, "invoke a command with an altered scheduling priority." I read this to mean that you could possibly start File Sharing with a lower processing priority, thus causing a reduced drain on system resources.
If anyone is savvy enough to try, please let us know.
-aaron
[Editor's note: See the comments -- nice is broken in the current release of Mac OS X, so this command will have no impact].
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010328205920773