Safari cannot open the page because it could not establish a secure connection to the server

Safari can’t open a website

Safari – slow page loads

Safari - Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues

Web Content not responding

Here is what Apple Support says about this issue.

https://support.apple.com/en-us/HT204937

macOS

Do you have any anti-virus software installed on your Mac?

Parental control set?

Is system time set correctly?

For more info:

https://support.apple.com/en-us/HT204098

Section 5: https://support.apple.com/en-us/HT201400