Hello Sudharsan,
Importing Very Large Data Sets : When very large data sets are imported in a single job.
Symptoms: The import job takes a long time to complete.
How to avoid this: Break a very large data set into multiple, smaller jobs for faster results. Consider import sets under 100,000 records as a guideline. For example, importing 10 sets of 100,000 records completes faster than one import of 1 million records even though the total data imported is the same.
http://wiki.servicenow.com/index.php?title=Import_Set_Performance_Best_Practices#gsc.tab=0
I hope this answers your question.
