It takes time to know what is inside a person. I think we all have a bias towards thinking that the way that someone communicates is a good proxy for how smart they are and what they’ve got going on in their brain. I don’t think you have to go any further than someone speaking a language they aren’t familiar with to know that isn’t true. I can sound very smart in English but I sound incompetent or dumb in Spanish.
It takes time to see their actions over time, how quickly they pick things up, and how they handle change to figure out what they’ve got going on inside.
Give everyone that time. Don’t write anyone off.