We ran a mongo config server failover test several weeks ago. What we did is to ramp up some load in the system and then we started to stop services on mongo config server in the sequence of master, slave1 and slave2.

Based on the following comments from mongo knowledge-base, the system should still function without performance degradation.
If all three config servers are unavailable, you can still use the cluster if you do not restart the mongos instances until after the config servers are accessible again. If you restart the mongos instances before the config servers are available, the mongos will be unable to route reads and writes.

However, the following findings from our load test is not what we expect according to mongo knowledge-base.

1. When one of the mongo-config server service was stopped the overall throughput drop from 1008.52 to 995.9
2. When two of the mongo-config server services were stopped the overall throughput drop from 995.9 to 652.
3. When all three mongo-config server services were shut down, all requests are failed.

Is this a known bug?