Well, I tried already (several times) but seems no impact.
I checked the logs at the time of the cache update request and just got:
08:40:58.359 T:545586692320 NOTICE: PLEX.artwork: Doing Image Cache Sync 08:41:02.321 T:545586692320 NOTICE: PLEX.artwork: Image cache sync about to process 8908 video images 08:41:03.919 T:545586692320 NOTICE: PLEX.artwork: Image cache sync about to process 1303 music images
I waited around 2hrs but no other message from PLEX.artwork
