I would like to confirm that updating Java fixed the spinning wheel for me. Before I had a spinning wheel whenenver I was dragging a reference to another group, now I can drag multiple references and it’s as quick as I expect it to be.
Java 8 Update 231
Endnote X9.3.1
Mac OS 10.12.6
Macbook Air End 2010
Credit goes to ScrippsOcean
And a note to Clarivate support: the fix was published by the above mentioned user back in May, yet no official update or even a note to update Java? Please improve support!