alacrity

alacrity \uh-LACK-ruh-tee\, noun:

A cheerful or eager readiness or willingness, often manifested by brisk, lively action or promptness in response.

As for his homemade meatloaf sandwich with green tomato ketchup, a condiment he developed while working in New York, I devoured it with an alacrity unbecoming in someone who gets paid to taste carefully.
-- R.W. Apple Jr., "Southern Tastes, Worldly Memories", New York Times, April 26, 2000

Arranged in long ranks, ten-, twelve-, or thirteen-year-old girls, pale and hollow-eyed, their pinned-back hair sprouting tendrils limp with perspiration, operated the machinery with such alacrity that arms and hands were a flying blur.
-- Patricia Albers, Shadows, Fire, Snow: The Life of Tina Modotti

So, I am sure that I was thrilled when I got the letter offering me the fellowship and equally sure that I wrote back to accept with alacrity.
-- Joan L. Richards, Angles of Reflection

Alacrity comes from Latin alacritas, from alacer, "lively."