I did the optional interview, and it went pretty poorly. My interviewer seemed tired, disinterested in what I was saying and moved on to other questions way too quickly. Whenever I tried to elaborate on my answer, he would cut me off by immediately asking the next question and now I’m worried that he might have thought I was being rude and interrupting him.
I know the interviews are obviously optional and are mostly used to demonstrate interest, but how much weight do they really hold? Would this interview ruin my chances? Should I ED2 to somewhere else?
My two cents is I would not worry about this at all, nor change any plans based on it. Probably the most selective position I ever got, I thought I bombed the interview. Afterwards I literally told everyone I knew that there was zero chance of getting the offer. And then I did.
In this case, I think all your experience is illustrating is that some people are just not great at being an interviewer. But you should trust that WUSTL knows that too.