[Samba] Security=user half as fast as share. Why?

[samba1.20.cdunham at spamgourmet.com]

I'm running Suse 9.2 (kernel 2.6.8-24) with Samba 3.0.20b

When I use security=user, my read/write performance is less than half the 
peformance with security=share.  Why?  There are no other changes, but 
performance is never better than half the speed.  This happens with WinXP, 
Win2K and Win98 clients, so I don't think the client configuration is the 
problem.  Network is gigabit, with Intel Pro/1000 adapters.

My smb.conf file is:

[global]
 workgroup = MYWORKGROUP
 netbios name = SERVER1
 wins support = yes
 domain master = yes
 local master = yes
 preferred master = yes
 os level = 65
 map to guest = Bad User

;security = share
 restrict anonymous = no
 server signing = Auto

[test]
 comment = test
 path = /home/users
 read only = no
 guest ok = yes