This option will decrease the amount of slowdown experienced at the end of large imports. The slowdown is partially caused by the need for WP All Import to read deeper and deeper into the file on each successive iteration. Splitting the file into pieces means that, for example, instead of having to read 19000 records into a 20000 record file when importing the last 1000 records, WP All Import will just split it into 20 chunks, and then read the last chunk from the beginning.