I was very tilted this week because I was failing 4s and 5s, then I did a few 9s on my main and it was very easy. The difference is the higher level the key, the more likely that you attract people that have actually dragged abilities onto their bars and are pressing them.
As a tank or healer I honestly haven’t found the difficulty of the dungeon any different from 4-9, the difficulty all comes from whether people in the group press defensives/interrupts/CCs or not.