First response time tells you how quickly customers hear back; time to resolution tells you how quickly their problem goes away. A team can be excellent at one and poor at the other.
Decide whether time spent waiting on the customer counts. Most teams pause the clock while a request is waiting on the customer, so TTR reflects the team’s work rather than the customer’s inbox. Like FRT, report it as a percentile, and split it by priority, because a password reset and a production incident don’t belong in the same average.