We have to investigate to know for sure, but if I had to guess it's probably in the storage engine/disk IO.
Surely your SSD has more than 800 IOPS though. Have you done any profiling?
Sorry, I misunderstood your question. In this specific benchmark the bottleneck is in the network and disk latency (since the benchmark sends out a batch of writes, waits for a server acknowledgement, which in hard durability mode means waiting on disk), and then sends out the next batch.
When we use a benchmark that doesn't bottleneck on latency (by adding more concurrent clients, or by using noreply) the ops throughput approaches theoretical IOPS throughput of the SSD.