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 Producer 1 Consumer 89.21 min.insync.replicas = 2 1 Producer 1 Consumer 298.99 min.insync.replicas = 1 Another user reported throughput falling from 147 MB/s on Kafka 3.4 to 58 MB/s on Kafka 3.9 with the same setting. The root cause Chia-Ping Tsai, a long-time Kafka contributor, identified the issue. It traced back to KAFKA-15583, titled "High watermark can only advance if ISR size is larger than min ISR." The high watermark (HW) is the offset of the latest message copied to all in-sync replicas.…