Throughput decreased when thread increased

[ats1999]

1 Thread

Running 10s test @ http://127.0.0.1:3000
  1 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.94ms    0.94ms  20.08ms   85.76%
    Req/Sec    20.35k     2.60k   27.62k    84.16%
  204487 requests in 10.10s, 39.00MB read
Requests/sec:  20238.28
Transfer/sec:      3.86MB

2 Thread

Running 10s test @ http://127.0.0.1:3000
  2 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.71ms  562.58us  11.89ms   87.36%
    Req/Sec    10.66k   569.07    14.23k    86.63%
  214321 requests in 10.10s, 40.88MB read
Requests/sec:  21215.08
Transfer/sec:      4.05MB

3 Thread

Running 10s test @ http://127.0.0.1:3000
  3 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.67ms    1.44ms  33.06ms   94.10%
    Req/Sec     7.20k     1.17k    8.64k    89.44%
  217227 requests in 10.10s, 41.43MB read
Requests/sec:  21503.67
Transfer/sec:      4.10MB

4 thread

Running 10s test @ http://127.0.0.1:3000
  4 threads and 100 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.71ms  619.95us  17.39ms   89.46%
    Req/Sec     5.33k   367.66     7.03k    82.43%
  214443 requests in 10.10s, 40.90MB read
Requests/sec:  21230.08
Transfer/sec:      4.05MB

You can see Req/Res decreases as we increase the thread. What could be the reason?