When I add a subdomain and look at it in DNS or DNS editor,

I noticed that in the parent domain,

there is a record created (ns) that points to the internal server name of the server that is running the DNS server.

The issue would be that if the records would be needed then the local server name would not be resolvable on the internet

Further.

when I add a domain or website it can take a long time before its added (server 2016, IIS 10, using DNS 2012+) .

The rest of the site is fast.