By default the CSV Import import the CSV file in batches of 100. That is what the progress bar shows. It time it increases in length, a batch of 100 from the CSV file was imported.
The change I made was I limited the importing to 25 per batch. So each time the progress bar increases in length 25 rows from the CSV file was imported.
The file will continue to be imported in batches until the entire file has been imported. It just is not possible to import hundreds or thousands of items into the database all at once. That would exhaust server resources and cause the site to stop working.
I hope this clears it up and does not cause further confusion. Let me know.