Me too. Reported this ages ago and it was dismissed as a “MacOS bug that needs to be resolved by Apple.” Kicking the can down the road. As an Apple seed tester, I don’t deny there are issues with macOS (I’m currently running 10.13.4, but EN8 has been nearly unusuable for months now) but I’m not seeing anything like this with any other software I use. This is truly an EndNote 8 issue and nothing I’ve tried (using a test library, for one) helped. It starts off ok and within minutes triggers a beachball with every click.
Please fix this.