This reproduces with the insert benchmark and 8 clients on a c3.xlarge host. The cpu-bound test inserts 80m docs and the database is mostly in memory. The io-bound test inserts 320m docs and the database is larger than memory.

Insert rates for CPU-bound test with 8 clients:

• average value is 54682 & 24198 for rc11 & rc12
• per-client rate starts at ~20k/second, drops to ~6k/s & ~3k/s for rc11 & rc12 by load end

Insert rates for IO-bound test with 8 clients:

• average value is 7973 & 6216 for rc11 & rc12
• per-client rate starts at ~20k/second, drops to 1063/s & 781/s for rc11 & rc12 by load end

More details are here