Why is domain user authentication taking so long after migration?

Did you demote the old domain controllers properly? If not, that could lead to some strange issues after migration. Also, ensure your new domain controllers are configured correctly in "Active Directory Sites and Services". Running a DCDIAG could provide insight into any problems. Just a heads up, it’ll show recent event logs that might not be directly related, but it’s worth a look.

Slow logins are often related to DNS issues. It's possible there's a script that's still pointing to a resource that doesn't exist anymore, especially if you have firewalls filtering traffic. I'd recommend checking your DNS configurations and making sure everything is pointing correctly to the new servers. That could really help speed things up!

DNS is usually the main culprit in these situations. If your clients are still pointing to the old DNS servers, they'll have trouble with logins. If you're using DHCP, make sure it's set up to direct clients to your new DNS addresses. That can often solve these kinds of issues pretty quickly.

You might want to check your Group Policy Objects (GPO) too. If you have a PowerShell script that's supposed to run at login but it's incorrectly configured—maybe placed in the batch scripts section—it could definitely slow things down.

Just to clarify, 'autontification' seems like a typo. Do you mean authentication? If so, it further points to DNS or GPO issues.