Update a single field when saving an entity Question: How do I update a single field of an entity? For example, lets say I have a user and want to update field_my_field. How do I just update that without using user_save?

My Specific Problem: 
I have a a bunch of user fields defined, and am trying to optimize the performance of my site. I am using views_bulk_operations, and my code is timing out (maximum execution time) because I am repeatedly calling user_save, and it's an expensive operation.

I am only updating a single field, and would like to optimize this code. Rather than calling user_save, which is taking 500ms to run, I'd like to update the field directly.

I attempted to use field_attach_update(), and that works a little better, but that's also updating all the fields. Is there any specific way just to update or save a specific field for an entity?

I'm not interested in batch operations, or anything at the moment -- I know how to do that, and can convert this into a batch so I don't run into execution timeouts. I just want to know how to save a single field for an entity.
Give a try with field_sql_storage_field_storage_write.

See this blog post with an example:

//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);