This ability's casting time reduction does stack with the Dark Arts reduction and any other casting time reductions (i.e. Fast Cast).
This ability also affects the recast time of the next spell, reducing it by 50%, and does not stack with any form of haste to lower it more. However, it does not ignore the slow status effect.
When wearing Argute Loafers and under the effects of the weather that corresponds with the spells element the -cast and -recast is increased by 10% (-60% cast and recast total). Was tested and confirmed.
The word alacrity means cheerful readiness, promptness, or willingness.