Asp.Net core async action in controller

Question

best practise is using async actions in API. OK:

    [HttpGet]
    public async Task<ActionResult<string>> Get()
    {
        await Task.Delay(10000);
        return "END";
    }

I call this code multiple. There are responses time: 1. 10s
2. 20s
3. 30s
.... responses are not async but sync? Why?

Why aren't all the responses in 10 seconds when it's async?