How 3 Lines of Code Caused a 10x Kafka Throughput Drop

In August 2025, a user reported that Apache Kafka v3.9.0 dropped consumer throughput by 10x. Other users reproduced it. The culprit was a configuration called min.insync.replicas , and the fix was three lines of code. The report Sharad Garg opened a ticket titled "Consumer throughput drops by 10 times with Kafka v3.9.0 in ZK mode." Ritvik Gupta ran controlled tests and traced the issue to min.insync.replicas . Setting it from 1 to 2 caused a massive drop: Test Message Rate Configuration 1 Produc