[Milhouse]

I wasn't referring to the thumbnails folder,
what i meant was the artworks stored in each movies & tvshows,
what if they're shared to multiple clients...

Yes, sharing the artwork stored alongside your media is absolutely fine - it's read once by each client then written into the cache (Thumbnails folder + Textures13.db)

compared to cached diskless userdata containing thumbnails folder unique to each client

As long as each client has their own copy of the databases in the Database folder (which includes Textures13.db) and their own copy of the Thumbnails folder, you shouldn't have a problem with a diskless setup. It will be slower than local storage, however.

It's strange, i just did some testing on 2 PC using diskless scheme

i used 52 tvshows stored in 2 A-E folders (intentionally duplicated to test the performance)
The first one has the artwork (posters, banners, etc) stored in NAS generated using filebot script
The second has the artwork fetched manually from Kodi and is stored within userdata folder

Both are restarted first before opening Kodi
Browsing the first one seems to be faster..
There's a noticable delay, especially in background poster for the second one..

But then again, this is only a small sample, can't tell if it'll be the same for larger sample and heavier network traffic

It all depends on what artwork needs to be cached. In theory the caching process will result in a slower user experience the first time an item of artwork is displayed, but there should be no delay when the same artwork is displayed on subsequent occasions.

As far as your test is concerned, a big factor is the state of the texture cache available to each client before you begin browsing around the GUI - if artwork needs to be cached the GUI will be slower, as it needs to cache the artwork before it can be displayed. You can check what artwork needs to be cached by running "texturecache.py nc tvshows". To pre-load the cache use "texturecache.py c tvshows"

If you want to test the performance of local (ie. NAS) artwork compared with remote (ie. internet based) artwork then delete both texture caches from your two clients (ie. Textures13.db and Thumbnails folder), restart Kodi and repeat your test - both clients will now be caching their respective artwork into their respective texture cache. Once artwork is cached, any subsequent delay is due to something else (CPU/GPU performance, storage/network performance, etc. etc.)