MariaDB Regression Traced to Mutex in Read Hotpath Introduced After 10.5

A performance drop in MariaDB versions after 10.5 has been traced to a mutex placed in a high-frequency B-tree read path. The lock forced threads to wait needlessly, causing latency spikes and throughput roughly three times lower than earlier releases. Standard microbenchmarks did not reveal the regression during the transition away from the partitioned LRU cache. An anomalous result in version 10.8.8, which matched version 10.4, prompted a closer investigation that identified the problem. Removing the mutex from the hotpath restored the lost performance, and the case highlights the need for broader benchmarking during major code changes.