Measuring response time using observables

Given Observable A representing incoming requests ...

interface IRequest
{
  string Id { get; }
}

IObservable<IRequest> requests;

and Observable B representing responses ...

IObservable<string> responses;

I need to measure the time it takes to produce a response. My current solution looks like below. This works but I'm not sure if this could be simplified or implemented more streamlined. I'm also concerned about the deviation between actual response time ([GEN]) and observed response time ([OBS]) which becomes quiet significant if generatedResponseTime is set to sub 50ms.

class Program
{
    public class Request
    {
        public string Id { get; set; }
    }

    static void Main(string[] args)
    {
        var generatedResponseTime = 100;

        var responseSub = new Subject<string>();

        IObservable<Request> requests = Observable.Interval(TimeSpan.FromMilliseconds(1000))
            .Select(x=> new Request { Id = x.ToString("d") });

        IObservable<string> responses = responseSub.AsObservable();

        var responseTime = requests
            .Where(x => !string.IsNullOrEmpty(x.Id))
            .Select(req => new { Id = req.Id, Start = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() })
            .SelectMany(req => responses
                .Where(reqId => reqId == req.Id)
                .Select(_ => new { RequestId = req.Id, ResponseTime = (int)(DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() - req.Start) })
                .Take(1))
            .Publish()
            .RefCount();

        requests.ObserveOn(TaskPoolScheduler.Default)
            .Subscribe(req =>
            {
                Console.WriteLine($"[GEN] {req.Id} = {generatedResponseTime} ms");

                Thread.Sleep(generatedResponseTime);
                responseSub.OnNext(req.Id);
            });

        responseTime.Subscribe(x => Console.WriteLine($"[OBS] {x.RequestId} = {x.ResponseTime} ms"));

        Console.ReadLine();
    }
}