Rendering a large amount of items can drastically decrease frame rate, notably in the GUI when opening inventories, containers/chests full of them. This can be very noticeable when arranging a large amount of items.

Seems to be that every single item rendered counts as a draw call, and considering the amount of items inside a full double chest and their separate layers, this can get up to dozens of draw calls, making rendering chest contents a resource intensive task.

A single item can have one of the next counted as a separate layer, which contributes to the lag considerably when stacked:

• Color layers (leather armor, tipped arrows, potions, etc).
• Pattern layers (banners, shields).
• Armor trims.
• Enchant glint (MC-233605).
• Item quantity label/number (MC-249635).
• Durability bar.

How to reproduce

1. Create a void world, place a double chest full of items and another one empty.
2. Open ALT+F3 and open both chests.
3. Compare the frame rate.