I reported this problem to Clarivate technical support, as my installation of Endnote had become unusably slow. They suggested that upgrading MacOS to the latest version of High Sierra (10.13.3) (I had previously upgaded to the latest version of Endnote X8.2 and it was still very slow) and this combination resolves the problem. Now Endnote functions as I would expect it to, Wow, night and day! Endnote is usable again.