PostgreSQL performance worse when adding CPUs
I often notice that PostgreSQL benchmarks run faster once more than half the CPUs are busy.  I think that what happens is Linux tries to keep the client and the server on different CPUs, which slows down the intense back-and-forth between them (which is the main thing your benchmark is benchmarking).  Once Linux is forced to run them on the same CPU, it gets faster.  

Although I haven't seen the effect to be nearly as large as you report here.