From parent ticket SERVER-69905:

The biggest issue is that this executor makes a fresh thread for each toplevel task. The contract of this scheduler is that a toplevel task gets a dedicated thread, but it is not promising a FRESH brand new thread, only a dedicated one. So when the toplevel task completes, the worker thread should be reusable. But we don't reuse it. We let it exit and just make new threads for each connection. This seems extremely wasteful and we should just pool these workers and hand out leases to them. Essentially this is not so different from an ordinary ThreadPool except that the threads are different kinds of objects from the stdx::thread objects managed by ThreadPool.