Issue originally opened at #2297817: Do not attempt field storage write when field content did not change and kept hostage for a year because of Drupal 8. Now that the Drupal 8 patch does not seems as obvious as it was and discussion is going further, I open a Drupal-7 only patch as per the backport policy states (actually it should be the other way around but the original issue is now too deep in Drupal 8 to change back).
This patch introduces a check in field SQL storage when updating or saving entities which prevents it from doing useless DELETE/INSERT when saving unchanged content. See details in the other issue, but this is a huge performance win when dealing with a high-frequency writes sites which has many fields (many start to less than 10 to see a difference).