Try it for yourself:

1. Get any item from the CREATIVE inventory.
2. Place 2 blocks side-by-side with a one block space open in between them.
3. Make sure you are holding an item in your hand, and break the first block.
4. Notice the split second delay before you broke the block.
5. Make sure you have NO item in your hand, and break the second block.
6. Notice there is NO delay when breaking a block with nothing in your hand.