The parameter user_data_source is specify the backend used to store subscribers and how this backend is managed. Very old versions started with only files to store subscribers. Now the only value supported for this parameter is

user_data_source include2

When using this value, subscription, bounces mangement and external includes are compatible. Updating external sources is performed by the task_manager deamon without performence issu when using web interface.

You should always use include2 for all lists

Make sure task_manager is running and just edit the parameter from the list config web interface (section data sources).