Too large batch size or timeout while waiting for server response while exporting data into cassandra

Question

I'm trying to import data from CSV files to cassandra using cassandra-loader: https://github.com/brianmhess/cassandra-loader

Cassandra-version: 3.1

Export data from cassandra into CSV files takes ~ 20 minutes which is ok. Import data using this command: cassandra-loader -f "path to files" -host "host" -schema "scema" -numThreads 40 takes more than 7 hours. If i try to increase -batchSize over than ~1.5k and -rate over than 50k than i get an error.

Error inserting: Batch too large
com.datastax.driver.core.exceptions.InvalidQueryException: Batch too large

Sometimes i get error like "Server responce timeout"

Timed out waiting for server response
com.datastax.driver.core.exceptions.OperationTimedOutException: [] Timed out waiting for server response
    at com.datastax.driver.core.RequestHandler$SpeculativeExecution.onTimeout(RequestHandler.java:772)

How can i fix it and speed it up?

Or maybe there is another way to import fast data from CSV into cassandra