If you were improving a system, how would you pick a KPI? Can you give me an example of a KPI you would keep an eye on, at Wealthsimple?

Interview question asked to Technical Program Managers interviewing at Wealthsimple: If you were improving a system, how would you pick a KPI? Can you give me an example of a KPI you would keep an eye on, at Wealthsimple?.