It seems actual individual performance is worth close to nothing in the way they calculate the rank gain or loss after a match, and even the actual score of the match doesn't seem to matter.
I lost a payload that went into sudden death, both matches were close. I performed well, had 2 gold for objective kill, and objective time, and 2 silver in elimination and damage as Zarya. I think 17 kills, 12 on objective or something like that. I lost 3/4 of a level.
I lost another payload, a total rout, we were absolutely destroyed...they won on attack, we didn't get the first point. I had an horrible game, 1 bronze medal only. I lost 3/4 of a level also.
I'm not even sure if anything but win-loss actually enters the equation, like a loss is 3/4 no matter individual performance.