[Python] Uvicorn workers greater than one apparently reduce performance by half

My main test app:

from starlette.applications import Starlette
from starlette.responses import PlainTextResponse
from starlette.routing import Route

async def homepage(request):
return PlainTextResponse("Hello world!")

app = Starlette(routes=[Route("/", homepage)])


Tests using wrk and bombardier:

uvicorn --host 0.0.0.0 --port 8000 --workers 1 --log-level critical main:app

wrk -t4 -c100 -d15s http://127.0.0.1:8000
Running 15s test @ http://127.0.0.1:8000
4 threads and 100 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 20.97ms 1.97ms 42.51ms 72.49%
Req/Sec 1.20k 75.01 1.33k 81.00%
71484 requests in 15.01s, 9.95MB read
Requests/sec: 4763.71
Transfer/sec: 679.21KB

bombardier-linux-amd64 -c 125 -d 15s http://localhost:8000
Bombarding http://localhost:8000 for 15s using 125 connection(s)
[====================================================================================================================] 15s
Done!
Statistics Avg Stdev Max
Reqs/sec 3554.25 1317.96 6167.59
Latency 35.11ms 4.23ms 59.66ms
HTTP codes:
1xx - 0, 2xx - 53452, 3xx - 0, 4xx - 0, 5xx - 0
others - 0
Throughput: 722.57KB/s


uvicorn --host 0.0.0.0 --port 8000 --workers 2 --log-level critical main:app

wrk -t4 -c100 -d15s http://127.0.0.1:8000
Running 15s test @ http://127.0.0.1:8000
4 threads and 100 connections
Thread Stats Avg Stdev Max +/- Stdev
Latency 41.52ms 1.58ms 59.52ms 96.24%
Req/Sec 603.89 33.36 720.00 73.50%
36094 requests in 15.02s, 5.04MB read
Requests/sec: 2403.45
Transfer/sec: 343.55KB

bombardier-linux-amd64 -c 125 -d 15s http://localhost:8000
Bombarding http://localhost:8000 for 15s using 125 connection(s)
[====================================================================================================================] 15s
Done!
Statistics Avg Stdev Max
Reqs/sec 2905.55 432.64 5376.82
Latency 42.99ms 2.20ms 74.43ms
HTTP codes:
1xx - 0, 2xx - 43671, 3xx - 0, 4xx - 0, 5xx - 0
others - 0
Throughput: 589.74KB/s


From now on, increasing the number of workers does not make any difference. The results with ab is different. Increasing the number of workers actually increases the performance. Now my question is why?

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