When running a command in the chat, it will delay for 5 seconds. (100 in-game ticks) for the first time everytime the world loads. Though, after you have ran a command for the first time since the world load, it won't delay anymore.

Detailed explanation:

If you create a new world, running a command will delay.

If you load a saved world, running a command will delay.

If you try to save and quit and load the game again, running a command will still delay.

It means that everytime you load a world if it's new or saved, running a command for the first time since will still delay.

Procedure: Make a new world or load a saved world, make sure the cheats are on.

After loading the world, try entering any command you want to the chat.

Expected: Command should run instantly.

Watch the attached video for visual explanation.