Thank you for your question and reaching out. I can understand you are having query\issues related to Slow GPO applying.
- Open Event viewer -> Go to Applications and Services Logs –> Microsoft -> Windows -> Applications and Services Logs -> Group Policy -> Operational -> Check events with the IDs 4016 and 5016 show the time when the processing of GPO application extensions starts and ends.
- Avoid costly WMI filters such as LDAP queries and Disable any long running startup scripts and The installation of software deployed through GPO.
- Verify DNS settings on client computers and it should be pointed to your DCs IP.
- Verify Date and time should be sync with your AD DC.
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