The System Policy Editor does not allow you to disable the "Automatically detect slow connections" policy because of an error in the Winnt.adm file.

You can edit the %SystemRoot%\inf\Winnt.adm file and locate:

POLICY !!SlowLinkDetectEnabled
VALUENAME "SlowLinkDetectEnabled"
END POLICY

Modify it as follows:

POLICY !!SlowLinkDetectEnabled
VALUENAME "SlowLinkDetectEnabled"
VALUEON NUMERIC 1 VALUEOFF NUMERIC 0
END POLICY

Configure and reapply the policy.

You can also edit the registry at:

Hkey_Local_Machine\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\SlowLinkDetectEnabled

and set this REG_DWORD to 0 (a value of 1 is enabled).