Cleaning an ECS-enabled cache may cause excessive CPU load
If a resolver cache has a very large number of ECS records stored for the same name, the process of cleaning the cache database node for this name can significantly impair query performance. 
This issue affects BIND 9 versions 9.11.3-S1 through 9.11.37-S1, 9.16.8-S1 through 9.16.45-S1, and 9.18.11-S1 through 9.18.21-S1.