Description

For some reason the send_offsets_to_transaction() call will be 50+ times slower when there is more than one message in the input topic of the transactor.

How to reproduce

I've made a reproduction demo that has two modes of operation. In one the issue is present and in the other it is absent.

The readme in that repository contains detailed information about the environment, the issue and the demo. There are log excerpts from the clients and the broker.

Checklist

Please provide the following information:

• confluent-kafka-python and librdkafka version (confluent_kafka.version() and confluent_kafka.libversion()): ('1.5.0', 17104896) ('1.5.0', 17105151)
• Apache Kafka broker version: 2.5.0 or 2.6.0
• Client configuration: see reproduction demo
• Operating system: Linux 5.8.14
• Provide client logs (with 'debug': '..' as necessary): see reproduction demo
• Provide broker log excerpts: see reproduction demo
• Critical issue: nearly unusable performance when the issue is present