title: Airflow scheduler is slow to schedule subsequent tasks
body: <p>When I try to run a DAG in Airflow 1.8.0 I find that it takes a lot of time between the time of completion predecessor task and the time at which the successor task is picked up for execution (usually greater the execution times of individual tasks). The same is the scenario for Sequential, Local and Celery Executors. Is there a way to lessen the overhead time mentioned? (like any parameters in airflow.cfg that can speed up the DAG execution?)
  Gantt chart has been added for reference:
  <img src="https://i.stack.imgur.com/MCE9d.png" alt="Gantt chart"></p>