Plugins like Datafeedr and WP All Import store information in wp_postmeta about imports and the SQL queries they use cause table scans to happen. When they are importing, they will check wp_postmeta for certain values (meta_value column on the table). By default, WordPress has no index on this table – probably because meta_value is a ‘text’ field meaning unknown size. You can still create an index on the first few characters however and massively speed up the majority of checks against this table.
Scalability Pro also provides an option to let you Defer Term Counts until 2AM. Term Counts involve recounting the number of items (posts, products, custom post types) within a category or taxonomy. When you are importing, this can mean term counts after every individual item. If you are importing 10,000 products then by deferring the term counts, you can run the count once when the import is finished rather than 10,000 times.
If you’ve been experiencing your imports becoming slower the bigger your database gets, Scalability Pro will definitely help you.