I've encountered two anomalies on MacOS 10.13 involving the "favorite folders" feature that was introduced in the February update to the desktop version of Lightroom. I believe one is a bug, but don't know whether it affects all catalogs where favorite folders have been designated or whether it was triggered by something in my computing environment. The other clearly isn't a bug because when you encounter it, Lightroom pops up a diagnostic error message that explains what is happening. But in my opinion it definitely is a misfeature.
The suspected bug involves the process of displaying your list of peviously-imported folders immediately after Lightroom launches. Before I designated some of my folders as favorites, the process was essentially instantaneous on my 2013 Mac Pro (six 3.5 GHz cores, 64 GB memory, catalog and image files all stored on a local flash device). After designating the favorites, the enumeration process was consistently taking about a minute and 20 seconds. I replicated this behavior with several back-up catalogs made after I started using the favorite folders feature. When I reverted to an earlier catalog which didn't include any favorite folders, the problem disappeared. Is anyone else who has selected favorite folders seeing this? It would be especially interesting—to me and, I suspect, the Adobe developers—if this behavior was also occurring on MS-Windows.
The misfeature involves simultaneously designating several folders as favorites. If you do this, and then subsequently try to mark some folders as no longer being favorites, that will only work if you are able to select those that were previously designated together. If you try to remove a subset or superset of the group as favorites, Lightroom pops up a diagnostic message saying that isn't permitted. Clearly from a usability perspective, it should be. Until and unless this is changed, if you're going to mark several folders as favorites simultaneously, make sure to remember which were included in the group to preserve your ability to back out the designation.