Why qp increased to bigger than 128, but Thoughput decreases instead?

As titile described, when qp upto 128, Thoughput down from 91 to 81. Then, although the value of qp increases later, the throughput still cannot reach the value as qp = 32.

qp Thoughput Gb/s
2 46
4 92
8 92
16 92
32 92
64 91
128 81
256 84
512 87
1024 88

PS:

  • mtu is 1500