time is of the essence
Time is a key consideration in this situation (often because time is limited or a specific deadline must be met). Has anyone called an ambulance? Time is of the essence with this man's injuries! We have to get these pages to the printer by 8, so work faster, people—time is of the essence!
Time is of the essence.
Timing and meeting all the deadlines are essential and required. (Often seen in contractual agreements.) The final payment is due on the first day of December, by midnight. Time is of the essence.