I use many different versions of samba and have for years on all kinds of hardware and different *NIX (solaris both sparc and intel, hpux, atleast a dozen different linux distros very old and new) and have not had any kind of performance problem as you suggest.
Have you looked at the debug logs?
What is your client and what is your server?
Are you using a HUB or SWITCH in your network?
Did you compile samba or install a package/rpm?
On your *NIX box have you looked at errors on the network interface with netstat to see if maybe there is a hardware problem at layer 1?
Also, did you hand-build your configs or use the webgui? I used to hand-build them and I almost always prefer CLI but samba's webgui really work swell. I just disable it in inetd.conf/xinetd when not needed.
Just some things to think about, I hope it helps.

Dave