Haste

Haste is a status effect which increases mining and attack speed. Negative levels will decrease mining and attack speed, similar to Mining Fatigue.

Immune mobs
Boss mobs are immune to Haste. Most mobs, while not immune, are unaffected by Haste as they cannot mine blocks.

Instant mining
With Haste and Efficiency, it is possible to mine some blocks instantly. Below are listed the Efficiency and Haste levels neccessary to instant-mine a block.

Commands
With commands, much higher levels of haste may be applied. Using these (or other mods that allow higher levels), the following blocks are instant-breakable.

Interaction with Mining Fatigue
Mining Fatigue III inflicted by elder guardians prevent the player from breaking any blocks regardless of the Haste effect. However, lower levels of Mining Fatigue can be overcome by Haste of level III or more, with a resultant reduced Haste effect.