I use Ember media Manager to get the artwork all straight for Kodi I then set the folders to the right type (TV, Movies etc but set them to local only (.nfo) for the info and artwork. I currently run kodi on a sony Android TV and a droidbox t8s plus, pulling the files from a NAS box, and they pull up all the artwork well, with little or no lag (The artwork is all in the movies/TVShows own folders).
I dont know if the boxes cache them locally as well through, never even throught about that.
I tried to run a sql backend (on the NAS) but never got it to work. I just have trakt loaded which i think syncs the watched show "ticks". (havent used the second box for a while)