Why is Microsoft Excel 2016 so slow when Internet connected
When I had a similar problem, disabling dropbox integration (I never realized I had enabled it) made a huge difference in performance, especially for otherwise trivial things in Office/Excel like copy and paste (which had become unbearable). 

System Preferences->Security &amp; Privacy->Privacy Tab->"Allow the apps below to control your computer"  (dropbox was checked, I unchecked it). Not sure exactly what functionality this took away from dropbox, but it still syncs folders fine, which is all I used it for anyway.

(I know this is thread is over a year old, but it's one of the first results in Google so I'm adding this for all the others who come along in hopes it may help them).