unless you collect data points over a relatively long period of time, it's hard to assess the competence of a developer. you might have one person on a team who works on bugs in ascending order of difficulty and closes many tickets every day. you might have another who works on high-priority difficult bugs, but takes several days on each one.

either person could be a valuable member of the team, but how do you say which one is more competent in a quick glance?