You may notice a little bit of slowdown in the UI as the thumbnails are initially regenerated. This is done on demand so it will be more of a little at a time as you scroll through your library and not a machine hanging for period of time as it deals with all of them at the same time. Also if you have any library items that are set with artwork that is no longer at the same location you will get empty art until you bring up the information window for that item and either refresh or manually select new art.
Basically the deal is that Kodi isn’t great about cleaning up dead artwork so under various situations there can be a large number of worthless files taking up space. There are various ways of dealing with this. The easiest and quickest is usually just dumping them and their database and let Kodi start over. Alternatives are using the Texture Cache Maintenance utility (which is more complicated to use) to selectively remove dead artwork or moving the thumbnail folder to a location with more space and doing a path substitution to redirect to that new location.