Probably alignment with mission. The siblings write a lot about it so read all that. I prepped but I missed the mark. I suspect because of doing too ordinary work so I didn't have examples that would make them think "damm this person gets us".
Anthropic has a tough alignment interview. Like I aced the coding screener but got rejected after a chat about values. I think they want intense people on the value/safety side as well as the chops.
Yeah high performance web used to be an art. Now it's find what you are doing that's stupidly wasteful that you did to ship fast, and stop doing that thing.
Your app could add almost no latency beyond storage if you try.