Endnote is not only slow on High Sierra, it is slow on any platform including Mojave. If you are in the beginning of an article do not hesitate to switch to bookends (especially if you are on MacOS). But if, like me, you are in the middle of your thesis and have no other options than continue to use endnote (which socks) then pray that the developers start all over again with a new version because it has become an Gaz factory which has inherited from all other previous versions bugs and slowness.
Flav