Fast to start or try something, as in This physician is quick off the mark in trying the newest medications. This expression comes from various kinds of races, where mark indicates the starting point. It was being used figuratively from the mid-1900s on.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.