If the database needs to do a full table scan, due to not having an index, it will take as long to pull the last day or so of data as it will the whole table. In this case an incremental load will not be of any use.
Is it that the data in the database is cleared down so that you need to persist old data to QVD?
The article I posted a link to above has lots on information on incremental loads. There are many ways of achieving them and the best for you will depend on your requirements.