Nothing. There is also nothing to encourage people from not taking forever. This metric worked well on my team until I got people who game it by pulling easier

Nothing. There is also nothing to encourage people from not taking forever. This metric worked well on my team until I got people who game it by pulling easier tickets and then sitting on them for a week. We still use the approach though. No approach survives a bad team. I could implement draconian rules, but that would just hamper the productive people.