Forms Authentication very slow.

Quick question I hope,

 I am currently using a site that uses Active Directory with forms authentication. Everything works perfect, except that it takes anywhere from 30 seconds to 3 minutes to logon using this method. If I try to use SQL then it only takes 1-8 seconds. Is there anything in code I can do to help speed this up? Or is it a hardware issue?

 Thanks for all your help!
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