Group Policy logon scripts are delayed by 5 minutes in Windows 8.1 - by design
This was briefly hinted at in the summer in a post about new group policy settings in Windows 8.1, but when I saw a post today spelling out what it actually meant, I was pretty surprised:
Warning!!!--Group Policy Logon Scripts Delays in Windows 8.1 - Group Policy Software - SDM Software | Group Policy Management & Administration Tools
That's the default value; luckily the aforementioned GP setting allows you to override that:
If you are using Group Policy-based logon scripts today to map drives or printers, set up registry or environment variables, etc., when you migrate your client machines to Windows 8.1, those logon scripts wonít run until FIVE MINUTES after logon has started. For some users, this could mean broken environments as processes or environmental setup steps that are expected to kick off at logon donít.
Computer Configuration\Policies\Administrative Templates\System\Group Policy\Configure Logon Script Delay