The mongoimport documentation still describes the tool as single threaded in 3.0:

mongoimport is single-threaded and inserts one document at a time into MongoDB. Custom import tools for data ingestion may have better performance for specific workloads.

However, this is no longer correct as at MongoDB 3.0 and the new numInsertionWorkers parameter is mentioned later on the same page: http://docs.mongodb.org/manual/reference/program/mongoimport/#cmdoption--numInsertionWorkers