i dunno if this has changed but last i checked every item slot in minecraft's GUI was also rendering one of these 3D items except that they're rotated so they look like 2D images and as far as i know there's no caching going on
meanwhile block game just caches 2d images to use as an icon for every item in the game at startup because uhh, hello?? why wouldn't you do that???
@gkrnours i feel like a slightly slower start up time to greatly speed up the game's rendering is a fair trade lol
also they could do it on-demand if they didn't want the startup time to be increased