If the player hits at something that isn't a block and holds the button while aiming next to a block, then moves the crosshair onto the block, there is a slight delay until they actually start swinging at and break the block. This is especially noticeable when instamining blocks.

Apparently this bug was introduced far back in 1.8, so it doesn't seem to be an intended mechanic from the cooldown changes of 1.9 (which also shouldn't affect block breaking).

How to reproduce

1. Place any easily breakable block, set your game mode to survival.
2. Put your crosshair next to the block looking at air, then press left click (default punch button).
3. Hold the button and quickly move your crosshair to the block.
4. Notice how there's a slight delay before the player actually starts breaking the block.

Expected result
After looking at the block, the player would start breaking the block without any delay, like in this clip.

Observed result
When moving the camera at the block, the player has a slight delay before they start breaking it, like in this other clip (requires slowing down the video to see easily).