To whom do you assign the task? Is this person able? Does he/she have the necessary skills and experience, the authority to carry it out?
If the person you first thought of or the one available is not the best fit, can you postpone until a better fitting staff can do?
If you hesitate, hesitate twice.
If you think the subordinate will take too long or may not be able to carry it out, hesitate to do it yourself. Ask yourself if you do yourself if the task is what is awaited from you/from your position.
May be you can do it better and faster, but next time you’ll face the same dilemma, you’ll still do yourself and your subordinates will never improve their skills.
Who is also about who will benefit from the task you delegate. Tell it your subordinate. Every adult needs to understand the purpose and for whom the efforts are made.