This is my experience as well. 15 keys are filled with people who just want to get this over with and can be very aggressive against anyone messing up and "wasting their time". Earlier keys are mostly people who either enjoy M+ or want to improve their ratings and thus accept others might also be on a learning curve. And obviously how much someone will perceive others as toxic has to do with their role. Healers but especially tanks are more likely to get abuse.