Placements and Feeders

I don't necessarily expect Riot to reach out to me on this topic, but I was hoping for some clarification. I am doing my placements for the first time ( now that I have played the game for around a year ), and I am incredibly frustrated. Almost every single game, I have dealt with terrible cooperation, DCs, and even some intentional feeding. I understand that placement is based on player performance, but what happens when a good performance becomes destroyed due to errors by my teammates? Is it really fair to judge a performance based on this? Maybe I am saltier then McDonald's fries, but I really would like to gain some clarity. (no pun intended)