when doing inserts with a single thread to an unsharded collection in a cluster of 4 shards, we observed a throughput of ~4k inserts/sec.
When doing inserts to a sharded collection, the overall throughput dropped to ~1.8k/sec. A drop of 55%. Per shard it was a drop by a factor of 10.
When doing inserts to a hashed sharded collection, the overall throughput dropped to ~1.5k/sec.

The sharded collection was well balanced and the throughput was well balanced across all shards (~400 inserts/sec/shard).

This was done on a single machine with an SSD, 4 mongod's running as a sharded cluster with single replica shards.

What could explain such a performance drop off?