Performance was implemented before Score was.
Score isn't a fair reflection in many situations either. It is biased towards shielders and pikers because they stay alive longer and soak up more proximity points, and very biased against ranged since arbalesters get 2-3 points per headshot which is silly.
Hey i had good score in that screen shot as an arbs man. Didn't even pick up a melee weapon for that screeny(and still went 31-5). But yea, score is AWEFUL for ranged, and if they get a high score, they are damn good, especially if using xbow only.
Also, I'm good? I'm only mediocre, though my last few battles I have done quite well(even when losing) making good performance(got a 7ish now I think)