Good afternoon. I installed Sierrab4 today and immeidately heard the fans and determined (Activity Monitor) that the Calendar processes were the issue. I just completed an extensive series of testing and discovered that the CPU rises to WELL over 200% and the fans speed up ONLY when I enable one of my specific accounts. This account is an O365 account. When I disable the "Calendars" checkbox all processes (and subsequently the fans) return to normal.
I began by first shutting down the caldenar process as described above by 'sparkplug37'. I then disabled ALL of the calendars from the Accounts Tab in System Preferences. I then removed the ENTIRE contents of the Calendars folder in ~Library. (Saved them to the desktop of course until I completed testing)
Now that the ~Library\Calendars folder was empty I re-launced the Calendar process using the load command. The Calendars folder began to refill with all of its folders and Caches. One by one (I have 6 accounts) I enabled the Calender checkbox in the Internet Accounts Tab in System preferences. Both my iCloud and all 3 of my Gmail accounts loaded normally and no issues. The moment I enabled the O365 account the issue once again started. I immediatley disabled the account and everything is running normally.
Not sure if any of your issues are O365 related, howver, mine is. As and FYI I do have another Exchange realated account (not O365). I tested it by enabling and did NOT have the issue. It is ONLY when I enable the O365 account.
Welcome thoughts and insight from others.