Upgraded to 10.13.1 this morning and ran some tests with a stopwatch. Still getting the same problem with build 12930 of EndNote x8.
Once I change the case of a title to sentence case, hit save, and try to click on another citation, it’s 10-12 seconds of inactivity and beachball before the selection field changes.
In EndNote x6 build 8318 the process is almost instantaneous on the same machine.