performance

Databases grow even when not a lot of new content is created. What happens is drupal mysql tables start to create overhead, especialy on tables with a lot of deletions. Cache tables, watchdog, access log, variable, ... . Sites that have been running for almost 5 years like this site for example create lots of overhead.
Optimize mysql tables

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.