The check to find dirty itemstacks and send update packets each tick can be very intensive as it checks the entire itemstack, including the entire NBT map. To save on this, 19/20 times we will simply compare the basic count/data/type. If for some strange reason the NBT of an item already existing in an inventory is changes, it will take up to 1 second to show, with an average time of half a second. This odd 0.5 second delay is far preferable to lag every tick, and shouldn't be noticed by anyone.
Thinkofdeath
md_5