Why is the initial connection time for a HTTP request so long?

The initial connection refers to the time taken to perform the initial TCP handshake and negotiating an SSL (where applicable). The slowness could be caused by congestion, where the server has hit a limit and can't respond to new connections while existing ones are pending. You could look into some performance enhancements in your Nginx configuration.

Use dig command to check you domain name resolution process. if return multi answer section, check these ips is valid.