Query active directory groups is suddenly very slow

We have a single web page that queries active directory to identify the group(s) to which a user belongs.  This page has worked fine for several years and suddenly today the page load time has gone from 1-2 seconds up to 15-20 seconds.  I've traced the execution of this page and the slowdown is in the part that queries active directory.

To the best of my knowledge nothing has changed on either the domain controller nor the web server.  The web server is Win2003 and the domain controller is Win2000.  Site traffic is within normal patterns.  All other applications on the web server are functioning normally (none use Active Directory though).  Direct activities on the domain controller (adding a user for example) don't exhibit any delay in response time.  Nothing in the event logs on either server that indicate a problem.

I'm looking for ANY suggestions where to look to resolve this problem.  The page in question is the frontend portal for all of our web apps so a 20 second load time is unacceptable.

If it helps, the code where the slowdown occurs is as follows (this is classic ASP):

  m_LogonUser = Request.ServerVariables("LOGON_USER")
  Set objUser = GetObject("WinNT://ourdomain.com/" & m_LogonUser)
      
  for each Group in objUser.Groups
    nNextElement = cint(nLastElement) + 1
    redim preserve sGroupArray(nNextElement)
    sGroupArray(nNextElement) = Group.Name
  next
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