The execution time in this method

public bool IsAuthenticated(String domain, String username, String pwd)
    {
        img = null;
        String domainAndUsername = domain + @"\" + username;
        DirectoryEntry entry = new DirectoryEntry(_path, domainAndUsername, pwd);

        try
        {//Bind to the native AdsObject to force authentication.

            Object obj = entry.NativeObject;
            DirectorySearcher search = new DirectorySearcher(entry);
            search.Filter = "(SAMAccountName=" + username + ")";
            search.PropertiesToLoad.Add("cn");
            SearchResult result = search.FindOne();

            if (null == result)
            {
                return false;
            }

            //Update the new path to the user in the directory.
            _path = result.Path;
            if (result.Properties["cn"].Count > 0)
           _userName = _filterAttribute = (String)result.Properties["cn"][0];

            
        }
        catch (Exception ex)
        {
            throw new Exception("Error authenticating user. " + ex.Message);
        }

        return true;
    }