Why Your Kafka Consumers Are Suddenly 10x Slower in v3.9.0

TL;DR In a minor release of Apache Kafka, consumer throughput dropped by 10x. This change was done to prioritize durability over throughput. The main star of the story was the new min.insync.replicas in the topic configuration. In versions before v3.9.0, it controlled when the broker accepted writes from producers using acks=all . Now, it can also dictate whether a message is able to be consumed by the consumers. This slight change caused a throughput drop of 10x for some Kafka users. Keep reading to find out why this change was made, and how to fix it, so that your production systems don't throttle. It All Begins In August 2025, a user Sharad Garg raised an issue on the Kafka Issue tracker. It was titled "Consumer throughput drops by 10 times with Kafka v3.9.0 in ZK mode". Other people validated his claim, and shared more information on the reproduction steps. Notably, Ritvik Gupta ran tests that pointed the blame on configuration min.insync.replicas . In his tests he showed that the