Calendar takes forever to sync a new event

Let's eliminate issues in your User by testing in a new User or Guest User.

Log in to new User and sign in with your Apple ID.

Only check Calendar for this test.

Give the Calendar a few minutes to sync up in the new User.

Add event and see if there is an improvement.

BTW, what version iOS? Have you upgraded to iOS 11?

Get help using iCloud Contacts, Calendars, or Reminders - Apple Support

Another user posted this to fix his sync issues with Calendar and his Google account:

I had this exact problem and solved it by deleting all items items in ~/Library/Calendars/Pending Events/ and ~/Library/Calendars/Incoming/. For good measure, you may want to delete all files starting with Calendar Cache in ~/Library/Calendars/ and the prefs (anything containing com.apple.ical) in ~/Library/Containers/com.apple.ical as well. This will resync the calendar with Google next time you start Calendar (in my case, I needed to restart Calendar once for the sync to finish).

~ is symbol for User.

Show User's Library

The User's Library folder is hidden by default. To unhide:

Option 1) Select the Finder in the Dock. Under Go in the Menu bar > hold down the Option key and you’ll see the Library.

Option 2) You can also select the Home folder in the sidebar of a Finder window.

Pull down the “View” menu and select “Show View Options”

Check the box for “Show Library Folder”

Read the steps in the article iCloud: Remove an iOS device backup from iCloud

You are signing both Mac and iOS with same iCloud id and password to sync .