SmartSubscribe Import is designed for batches of 10,000 or more Contacts and is fast and smart. In a comparison of two identical data sets consisting of 67782 records with no custom data fields, Normal subscribe completed in 36 minutes while Smart Subscribe completed in 4 minutes. If the import has 50,000 or fewer records, high-speed mode is used, making use of advanced database commands to speed the import.
One other important feature of SmartSubscribe is the ability to clear data fields. Using SmartSubscribe, Contact and Custom Data will be blanked out if the field for a particular contact in the import file is blank or empty. If you do not wish to clear data, then you must either submit data for every field or use the Normal subscribe, which will be slower.
SmartSubscribe is enabled by default in the Import Contact wizard and should be used for all imports unless you have made a business decision to use the slower Normal import method so as to prevent data from being cleared.
To Import using SmartSubscribe, follow the steps in Import Contacts, and be sure to select the 'Use SmartSubscribe' option on the first page of the Import wizard.