Meta Box Custom Table Addon 

MB Custom Table is an extension of Meta Box plugin that helps you to save data to a custom table instead of the default post/user/term meta table. All meta values are saved in a single row, and each column will be a meta key. This reduces the number of rows in the database which can cause a performance issue when the data grows. And let you have all of your data in one place, so you can easily view, edit, import, export it.

Meta Box Custom Table Addon GPL Features

• Uses WordPress recommended method to create custom tables, which allows you to upgrade table structure anytime.
• Provides a simple API to define table structure.
• Works with any meta types: post meta, user meta, term meta, comment meta.
• Provides an easy way to integrate with your existing meta boxes.

Why custom table for custom fields?

By default, WordPress uses metatables for storing the metadata (custom fields). In order to satisfy a wide range of needs, WordPress uses a flexible database structure, where each custom field is stored as a row with 4 columns: meta_id, post_id, meta_key and meta_value. While it seems to be fine at first, when your data grows, the number of database rows increases very fast.