When the "sneak" (crouch) button is held, the Rapid Use System does not work for any interaction, especially the Constant Placing System (seen in MCPE-51825). 

Steps to Reproduce (in Creative mode) 

1. Obtain a block that is not part of the Constant Placing System (walls, doors, or plants can be used here) 
2. Fly upwards a few blocks (make sure you can still target the blocks below you) 
3. Then do the following: 

• Fly forwards while sprinting 
• Hold the "jump" and "sneak" buttons 
• Look down 
• Hold the "use" with the block from earlier in hand 

Here, the block is only placed every 0.25 seconds. If the "jump" and "sneak" step above were removed, the block would be placed rapidly.