foo.js

class Optimized extends React.PureComponent {
  render() {
    return this.props.children
  }
}

const SomeConsumer = ({ slice, children } => (
  <ActualConsumer>
    {(state) => (
      <Optimized slice={state[slice]}>
        {children(state[slice])}
      </Optimized>
    )}
  </ActualConsumer>
))

<SomeConsumer slice="user">
  {user => (
    // only gets updated user when it needs to
  )}
</SomeConsumer>