1
general::
Phrase(s): quick off the mark
quick starting or reacting. (Compare this with slow off the mark.) • Boy, you were quick off the mark there! • If you can be really quick off the mark, you can soon find the answer.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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.
American Heritage Idioms