Low Throughput of AWS Kinesis

I use python's boto.kinesis module to write records to AWS Kinesis. The maximum throughput that is reached is about 40 puts/sec. However, according to the Kinesis FAQ:

Each shard can support up to 1000 PUT records per second.

So my current approach reaches only 4% what is theoretically possible, which seems terribly low.

Does anyone have an idea how the throughput can be improved?

Setup: The Kinesis Stream is an instance with one shard. The producer is on a dedicated AWS EC2 instance (t3.medium) in the same region as the Kinesis Stream. It creates strings of about 20 characters lengths and sends them to the Kinesis Stream via boto.kinesis.Connection.put_record("my_stream", my_message).

Simplified code:

from boto import kinesis
import time

connection = kinesis.connect_to_region(REGION)
stream = connection.create_stream("my_stream", shard_count=1)
time.sleep(60)  # wait a minute until stream is created

for i in range NUM_MESSAGES: 
    my_message = "This is message %d" % i
    connection.put_record(my_message, "my_stream", "partition_key")