1. If a component changes state and its parent doesn't, then the other children aren't considered so it would be O(1). If the parent rerenders, then yes, we would consider each child and it would take O(n).

2. Yes, it would. You would really have to go out of your way to make that happen, though. React has a feature for batching state updates together to prevent unnecessary rerenders if all of the updates occur together. Beyond that, it's rarely an issue.

If you have a particularly long list you can use a tool like react-virtualized (https://github.com/bvaughn/react-virtualized) so that only the onscreen rows are rendered at all.