This issue was introduced into v1.8.1 in a recent change from a pull request on github.

This change intended to introduce thread affinity to the socket pool but the result was that it caused a failure to issue new sockets to existing threads and fully utilize the maximum socket pool size.

Threads attempting to checkout sockets from the pool were getting blocked and being forced to wait for an existing socket even when the total pool size was well below the max size value.