Replaceitem every tick makes it nearly impossible for the player to switch their hotbar slot.

Release_453415
Steps to reproduce:
Place down the following commands:
REPEAT: /replaceitem entity @p slot.hotbar 0 snowball 1

Set repeating chain to always active. Then try pressing any of the number keys. Player has to repeatedly and continuously press in order to switch the slot.

My guess is that for some reason the /replaceitem command is temporarily disabling the player's ability to change their hotbar slot.