I am seeing the same behavior in version 1.9 on Xbox One. Playing two player split screen. When either player opens inventory, game lags for both players.
Moving items from inventory to the quick bar, or vice versa, the game also lags.
We also noticed that if one of us is carrying a Shulker box, the issue is even worse. Both players are wearing a full set of enchanter armor and carrying multiple enchanted items as well. If we open a chest full of non-enchanted things, the lag does not occur. This behavior matches the original posters notes on how to recreate the issue.
CWios
I am seeing the same behavior in version 1.9 on Xbox One. Playing two player split screen. When either player opens inventory, game lags for both players.
Moving items from inventory to the quick bar, or vice versa, the game also lags.
We also noticed that if one of us is carrying a Shulker box, the issue is even worse. Both players are wearing a full set of enchanter armor and carrying multiple enchanted items as well. If we open a chest full of non-enchanted things, the lag does not occur. This behavior matches the original posters notes on how to recreate the issue.