How do you track your team's performance?

I'm a new IT manager, and I'm working out how to track the performance of my team. We use ZenDesk for ticket tracking, and basically all of their work is done as tickets (helpdesk, sysadmins, QA, business analysts). I've been thinking of setting up SLAs for different ticket types/priorities, and having a time that tickets must be responded to and completed based on the kind of ticket and its priority. Performance is basically what percent of tickets met the SLA.