Keys are also essential to retaining an element identity, which is structurally more important than performance, because 1) code may have a reference to an elem

Keys are also essential to retaining an element identity, which is structurally more important than performance, because 1) code may have a reference to an element and think that it relates to some data, 2) an element may have an explicit state (like focused, animating, etc). Keys are a hugely leaking abstraction, but are inevitable when you bridge a declarative immediate mode rendering into a stateful one.