Me, too. Upgraded my IPhone 15 Pro to iOS 18 & immediately had problems with the calendar as you described. Also eats up my battery (drained it completely < 3 hours at work). I also have NordVPN running. Didn’t sweat it too much at the time because my iPhone 16 Pro was due to arrive. But it too has the exact same issues.
My calendar is useless to use on any format…I run a combo of iCloud (family shared and not) plus my work’s exchange account. I’ve tried disabling combinations w/o success. I need a new update, I guess. I’m now venturing out to 3rd party calendar solutions. By far and away my worst transition between iOS updates and/or new phone upgrades. iPad, M4 Pro 11” was also upgraded and experiencing calendar freezing issues too.
I might need to see about downgrading my new iPhone 16 Pro to iOS 17, but that worries me since my new phone was obviously designed with iOS 18 for its operating system.