Using older versions of Outlook for Mac for personal email and calendar
I have exactly the same problem and after scouring the internet, resetting SMC, NVRAM, PRAM and deleting other apps that may be causing the issue I have narrowed it down to Outlook.
This needs a resolution soon, the issue is obviously with Outlook, not all these Macs that people are having the same problem with.
My spec:
Mac (Retina 5K, 27-inch, Late 2015)
3.2 GHz Intel Core i5
16 GB 1867 MHz DDR3
AMD Radeon R9 M390 2 GB
Mac OS 10.14.5
Outlook version 16.26