If you want to save only a specific field on your entity instead of saving the whole entity you can. In a submodule of the fields module you ll find a module called field_sql_storage module. In that module all functions available for the sql storage backend can be used. One of them is field_sql_storage_field_storage_write, which you give your entity, type, operator (update, insert) and which fields to save.
<?php
//Get the id of your field
$name = 'name_of_your_field';
$info = field_info_field($field_name);
$fields = array(info['id']);
//Execute the storage function
field_sql_storage_field_storage_write('model', $entity, 'update', $fields);
?>This can be a huge gain in performance since you only save the specific fields needed instead of saving the entire entity with all its fields.
Links
[1] https://dominiquedecooman.com/blog-topics/fields-0
[2] https://dominiquedecooman.com/blog-topics/performance
[3] https://dominiquedecooman.com/blog-topics/entities
[4] https://dominiquedecooman.com/blog-topics/d7tip