In Creative mode, when you break a bunch of blocks, there's usually a small delay before the next block breaks. Then sometimes some blocks will have about no delay and will break immediately after the previous one.

What I expected to happen was...:
Blocks to have about a 0.5 second delay before they break

What actually happened was...:
Some blocks break immediately after another

Steps to Reproduce:
1. In Creative mode, place a high stack of gravel/sand
2. Go to the bottom of the stack
3. Hold down on the lowest block to break it and when it breaks, keep holding
4. The rest of the gravel/sand blocks will break extremely quickly and the stack will fall much faster.