### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

It seems like whenever I open a connection to `localhost:4566` (4566 being exposed from the localstack container), for the next ~1 second any HTTP calls will have delayed responses. If I open 10 connections, the delay will be 10 seconds.

### Expected Behavior

Multiple connections can be opened and don't impact other requests until a certain connection limit.

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

1. run localstack: `docker run --rm -it -p 4566:4566 --name ls localstack/localstack:0.14.0` - I managed to reproduce it with versions as old as 0.12.0
2. run something like this to see how long curls take inside the container: 

```bash
docker exec -it ls /bin/bash

# inside the container:
while true; do time curl localhost:4566; sleep 0.1; done
```

3. open connections with telnet:

```
telnet localhost 4566
```

4. observe extended response times in the curl loop. For me it's ~1s extra per open connection (I'm simulating opening more connections with a JVM application opening sockets as I request).

### Environment

```markdown
- OS: macOS / ubuntu, tried both x86 and arm64 macs, and an arm64 linux
- LocalStack: 0.12.0, 0.13.1, 0.14.0
```


### Anything else?

This surfaces itself as extremely slow responses whenever you have e.g. a JVM application that keeps a cached connection pool. Even 10 connections in a pool are enough to cause consistent delays every couple seconds (which I think is after the connections are closed due to the defaults on my client), under no additional traffic targeting localstack.
response._content = json.dumps(json_safe(listener_result))
            response.headers["Content-Type"] = APPLICATION_JSON
            response.status_code = 200
            break
        elif isinstance(listener_result, Request):
            
            
            modified_request_to_backend = listener_result
            break
        elif http2_server.get_async_generator_result(listener_result):
            return listener_result
        elif listener_result is not True:
            
            code = listener_result if isinstance(listener_result, int) else 503
            response = Response()
            response.status_code = code
            response._content = ""
            response.headers["Content-Length"] = "0"
            append_cors_headers(request_headers=headers, response=response)
            return response
    
    headers_to_send = None
    data_to_send = None
    method_to_send = None
    if response is None:
        headers_to_send = handler_chain_request.headers
        headers_to_send["Connection"] = headers_to_send.get("Connection") or "close"
        data_to_send = handler_chain_request.data
        method_to_send = handler_chain_request.method
        request_url = get_proxy_backend_url(handler_chain_request.path, run_listeners=True)