bartle reports high contention for the replication coordinator mutex in heavy insert workloads with w:majority writes, which leads to low CPU utilization and bottlenecking on a synthetic resource (the mutex). This is problematic on deployments with many cores, but can even be a problem on 16-core machines, as he mentions in a comment on another ticket.

Shortening the critical section under the mutex in setMyLastAppliedOpTimeForward and particularly in _wakeReadyWaiters_inlock is one possible approach to mitigating the problem. Finer grained locking around waiters might be another.