The data_backup command is more light-weight, and the data_restore operation is heavier. The more un-committed changes there are, the heavier the data_restore operation will be.
The data in the CQL CSV files are compared to the data currently in the system database (Cassandra). It then uses the CQL write operations to write all the changes needed in the system database to make it match the data in the CQL CSV file. If there is much data that needs to be changed to make the system database match the CQL CSV file, the operation becomes very slow and heavy.
Make it a habit to always commit all configuration changes, by doing a data_backup at once. During peak traffic, it is not recommended to do any bigger configuration changes. At any time, when a bigger configuration changes is made (with respect to traffic disturbance), a data_backup should be made.