|This spell speeds the actions of a creature. Hasting yourself will increase your magical contamination.|
Haste is a level 6 Charms spell which gives you the Haste condition, causing you to perform all actions 50% faster (2/3 normal time). Increased spell power increases the spell's duration. A potion of speed produces a similar effect.
Repeated use of the spell to maintain the haste condition causes magic contamination.
Note the difference between this spell and Swiftness, the level 2 Charms/Air Magic spell which speeds your movement only, at the cost of Stealth and trap detection. Note also that Haste and Swiftness stack.
This spell functions identically for monsters, with one important exception: monsters that only possess the basic spell Haste cannot haste their allies. There exists a separate spell Haste Other, which allows monsters to do just that. However, fairly few monsters possess it, and even fewer possess both it and Haste, some ogre magi being among the few examples.
The separation between the two monster spells, and the resultant inability of ancient liches and deep elf sorcerers to haste their summons, is more of an artifact of the way monster spellcasting is implemented than a mercy on the part of the developers.
Prior to 0.8, Haste made all actions 100% faster. In very early versions of Crawl it was a level 8 spell: magic contamination did not yet exist, meaning that this spell could be spammed with no downsides other than food costs, so it merited being a higher level.