I was introduced to this while in Saudi Arabia in the early 80’s, I lived in Iran in the mid 70’s and did not notice any major racial problems. Religion and big noses (from an area near the Caspian sea) were problems. I admit that I obtained near zero proficiency in Arabic or Farsi (check out what the farsi word "kos" means.
Is kus also some kind of racial slur in Persian? I assumed it meant the same thing as it does it Arabic.