I constantly get a never-ending spinning beach ball in Safari on tabs that have Google Calendar loaded I keep seeing the spinning beach ball on any tab that has Google Calendar loaded. It doesn't happen right away, it typically takes a while before it starts beach balling. The calendar works fine the whole time. Restarting Safari makes it go away.

Does anyone know how what could be causing this?

I used to get a message saying that the page has been reloaded because of a problem. Not sure if that's related but I haven't seen that with Safari 12 yet even though I have seen this behavior with that version.

Using Safari 12.0 now but this has happened with the last 2 or 3 major releases at least. I thought it could be an extension but Safari 12 has disabled all of my extensions except for 1Password.
I experienced this same problem. I have had some success by changing my Google Calendar alerts from "Alert" to "Desktop Notification".