From the mithril docs you linked to:

"You should only use onbeforeupdate to skip diffing as a last resort. Avoid using it unless you have a noticeable performance issue."

From the React docs (https://reactjs.org/docs/react-component.html#shouldcomponen...):

"Use shouldComponentUpdate() to let React know if a component’s output is not affected by the current change in state or props. The default behavior is to re-render on every state change, and in the vast majority of cases you should rely on the default behavior. [...] This method only exists as a performance optimization."

Both framework developers are telling users the same thing.