If you put multiple (i.e. 10) shulker boxes full of items in your inventory the FPS of the game, especially when opening and moving items around in your inventory, drops considerably. This does not happen on the old XBox One version of MC.
This happens on the X Box One X, so I dread to think how it is on a normal Xbox One console.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-27167
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