Time to market(TTM) is the best metric in my opinion. This is the metric which is easily understandable by anyone. But to make it clear TTM - is a time which a task takes to reach a customer. In software development world TTM means the time from when we move tasks from backlog to in progress till release to the customer. I like this metric because it's hard to cheat.
To improve this metric team can start splitting tasks to smaller subtasks. But this is great - small tasks improve estimation and prevents huge problems on the project.
To improve this metric the team can start making frequent releases. But this is great again because this is a direct way to continuous delivery.