Description: The 1.16 update has made it so that when clicking "e" to open the inventory (or any other button on different devices), the client needs affirmation from the server before the inventory can be opened. On higher-ping servers and multiplayer games, this causes a noticeable, even sometimes substantial latency. It is incredibly annoying to want to open your inventory to quickly move items, due to this issue, that is no longer possible to do swiftly because of the delay. PVP has been made very difficult and infuriating because of this unintended consequence of porting inventory to the server side.

If you try to open your inventory too many times in a certain interval, the inventory will not open in correspondence with the amount of times you attempted to open your inventory. Sometimes, the inventory will open and instantly close, or even crash the game. 

See MCPE-83980

The bug report above relates to this issue, but note that this report is not a duplicate.

Expected behavior: When attempting to open the inventory, it opens instantly and has no delay, and does not crash the game in rare instances.

Actual Behavior: Opening the inventory in multiplayer games and servers causes an annoying delay, sometimes the inventory does not open, or sometimes crashes your game entirely.

https://www.youtube.com/watch?v=5TaVQVfaJN4

See the video above. Notice how my inventory opens and instantly closes on its own.