Haste

Haste is a status effect that increases attack speed by 10% per level and increases mining speed by 20% per level. Negative levels 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 Instant breaking most blocks regardless of the Haste effect, Along with a huge decrease of mining speed. However, lower levels of Mining Fatigue can be overcome by Haste of level III or more, with a resultant reduced Haste effect.