The sstableloader is the fastest way to import data into Cassandra. You have to write the code to generate the sstables, but if you really care about speed this will give you the most bang for your buck.
This article is a bit old, but the basics still apply to how you generate the SSTables .
If you really don't want to use the sstableloader, you should be able to go faster by doing the inserts in parallel. A single node can handle multiple connections at once, and you can scale out your Cassandra cluster for increased throughput.