download speeds differeing...why?..

I have an xp computer on a windows 2003 network as a member of the AD domain...

when i download a file from our FTP site located on our company WAN , the
speed is OK,,,

But when i download that same file from xp computer that is NOT a member of
the
AD domain, i take about 3 times longer....

can you point me in the direction to a kb article or other doc explaining
what is happening behind the scene?... im suspecting some type of
authentification goin on ....

thanks..
rob