I see that there's a setting for wiredTigerConcurrentReadTransactions. We appear to be sometimes hitting this. I'm not really sure why this exists, but it appears to potentially be artificially capping throughput? If you have a really beefy machine, then you should be able to handle more concurrent read transactions? Is there any reason we shouldn't raise it? Does it increase contention to a point where things slow down more than they are sped up?
I see a brief reference to being able to change it in the docs (http://docs.mongodb.org/manual/reference/parameters/), but it's not clear to me if I should or what the implications are.