Move hash to permanent storage

Problem/Motivation

As a follow-up #3562989: Implements RevisionLogInterface for Instance entity. We are calculating the hash everytime we need (for display in LogsPanel or to check data changes):

class Instance extends ContentEntityBase implements InstanceInterface {

  public function getPublishedHash(): ?int {
    $published_data = $this->getBuildablePlugin()->getSources();

    return $published_data ? self::getUniqId($published_data) : NULL;
}

The logic is simple but it seems costly.

Proposed resolution

Because data is more often retrieved than saved, can we calculate the hash when we save the data instead ?

It will increase logic complexity (we need to save in both the live instance and in the permanent "published" storage) but it may increase performance.

How can we evaluate this performance gain, in order to decide if it is worth the hassle?