Why don't this await asyncio.sleep work?

Questions and Help
#1 
from sanic import Sanic
from sanic.response import text
import asyncio
app = Sanic("app")
@app.get("/")
async def async_handler(request):
    await asyncio.sleep(2)
    return text("Done.")

start with sanic server
Then I start two browser tabs and go to http://127.0.0.1:8000/ but they run after each other and not in parallel.

                                Sanic v25.3.0                              │
  │                   Goin' Fast @ http://127.0.0.1:8000                   │
  ├───────────────────────┬────────────────────────────────────────────────┤
  │                       │      app: app                                  │
  │     ▄███ █████ ██     │     mode: production, single worker            │
  │    ██                 │   server: sanic, HTTP/1.1                      │
  │     ▀███████ ███▄     │   python: 3.12.3                               │
  │                 ██    │ platform: Linux-6.8.0-31-generic-x86_64-with-g │
  │    ████ ████████▀     │           libc2.39                             │
  │                       │ packages: sanic-routing==23.12.0               │
  │ Build Fast. Run Fast. │                                                │
#2

The error was that my browser was blocking
try http://127.0.0.1:8000/?a=a and http://127.0.0.1:8000/?a=b