When you enable netlogon.log for the affected clients, you will see:
07/12 14:42:29 [MISC] DsGetDcName function called: Dom:NT-AUTORITÄT Acct:(null) Flags: DSP
07/12 14:42:29 [MISC] DsIGetDcName: Ignore single label DNS domain name NT-AUTORITÄT
07/12 14:42:29 [MISC] NetpDcInitializeContext: DSGETDC_VALID_FLAGS is c01ffff1
07/12 14:42:29 [MAILSLOT] Sent 'Sam Logon' message to NT-AUTORITÄT[1C] on all transports.
...
07/12 14:42:35 [CRITICAL] NetpDcGetNameNetbios: NT-AUTORITÄT: Cannot NlBrowserSendDatagram. (1C) 53
07/12 14:42:35 [MISC] NetpDcGetName: NetpDcGetNameNetbios returned 1355
07/12 14:42:35 [CRITICAL] NetpDcGetName: NT-AUTORITÄT: IP and Netbios are both done.
07/12 14:42:35 [MISC] DsGetDcName function returns 1355: Dom:NT-AUTORITÄT Acct:(null) Flags: DSP
When the customer creates a SCHANNEL.ETL file for the client while connecting the SSTP, he will see:
[1]0258.0290::07/12/2011-12:42:29.057 [sslproto]New cred:00000000003A5410, Protocol:80000000
[0]0258.0290::07/12/2011-12:42:35.945 [sslproto]GetUserName FAILED: 0x54b
[0]0258.0290::07/12/2011-12:42:35.946 [release]==============================================
So the delay is introduced by the German Name for "NT Authority".
This is reported in:
http://bugcheck/bugs/Windows8Bugs/415353
http://bugcheck/bugs/WindowsSE/384979