RabbitMQ: AsyncEventingBasicConsumer vs. EventingBasicConsumer

I have been doing a small sample application to consume messages on a queue in RabbitMQ.

The code should read the message an call a REST API (here replaced with a Task.Delay):

static void Main(string[] args)
{
    var factory = new ConnectionFactory
    {
        Uri = new Uri("..."),
        DispatchConsumersAsync = true
    };
    var connection = factory.CreateConnection();
    var channel = connection.CreateModel();

    var consumer = new AsyncEventingBasicConsumer(channel);
    consumer.Received += async (model, eventArgs) =>
    {
        Console.WriteLine("Doing a fake API call...");
        await Task.Delay(2000);
        Console.WriteLine("Done with fake API call!");
        channel.BasicAck(eventArgs.DeliveryTag, false);
    };
    channel.BasicConsume("myQueue", false, consumer);
}

When I run this application with five messages on the queue I get the following result:

The messages are processed one by one and with the 2 second delay it takes ~10 seconds. I would have expected to see five lines with Doing a fake API call... followed by five lines of Done with fake API call! with a total time of ~2 seconds.

When doing the synchronous version I see the exact same result - which was expected:

static void Main(string[] args)
{
    var factory = new ConnectionFactory
    {
        Uri = new Uri("...")
    };
    var connection = factory.CreateConnection();
    var channel = connection.CreateModel();

    var consumer = new EventingBasicConsumer(channel);
    consumer.Received += (model, eventArgs) =>
    {
        Console.WriteLine("Doing a fake API call...");
        Thread.Sleep(2000);
        Console.WriteLine("Done with fake API call!");
        channel.BasicAck(eventArgs.DeliveryTag, false);
    };
    channel.BasicConsume("myQueue", false, consumer);
}

My question is: What is the difference in using the AsyncEventingBasicConsumer compared to the EventingBasicConsumer? And: Is there a way of making the consumer process other messages while awaiting work for previous messages?