be slow off the mark

To be slow to react or take action. He's too slow off the mark to be an effective emergency responder. You were a little slow off the mark on your last line. Come in a little bit sooner next time. You won't make it as a goalie in this league if you're slow off the mark like that.

be ˌquick/ˌslow off the ˈmark

be quick/slow to do something or understand something: You have to be quick off the mark when you answer a newspaper advertisement for an apartment. ♢ Jenny was rather slow off the mark, and they had to explain the joke to her.

In athletics, your mark is your starting point in a race.