While it looks a lot like celery, celerity means something that vegetables are not — fast moving. Gossip often travels with celerity, as do children trying to get to a big plate of cookies.
When you see the word celerity, think accelerate, like a car when you step on the gas. Both words share the Latin root celer which means swift. You may want to study for exams with celerity, but using a slower, more methodical approach will often yield better results.