Symptom
In some cases it can take longer to truncate data from a class than it takes to write the same amount; for example 5 minutes to truncate, but only 1 minute to load.
Cause
If you set the writer to truncate using a log in that is not the owner - even though it still has permission to insert, update, delete - then it deletes records instead of truncating the table, and that is slower to accomplish.
Resolution
In this scenario, you should log in as the owner of the table in order to truncate it.