Why is my Initial call in RestSharp really slow? but others after are very fast

It's most likely the network settings causing this problem. I recently had the same issue and it turned out that when using HttpWebRequest or RestSharp it was trying some auto configuration to look for a proxy server.

Open the network settings in Internet Explorer and disable auto configuration for the local network. In my case this resolved the delay for the first request in RestSharp as well.

I had attempted @skrause's answer, but it wasn't work for me. I spend much time, and finaly I solved it. This my sulotion.

public class SimpleWebProxy : IWebProxy
{
    public ICredentials Credentials { get; set; }

    public Uri GetProxy(Uri destination)
    {
        return destination;
    }

    public bool IsBypassed(Uri host)
    {
        // if return true, service will be very slow.
        return false;
    }

    private static SimpleWebProxy defaultProxy = new SimpleWebProxy();
    public static SimpleWebProxy Default
    {
        get
        {
            return defaultProxy;
        }
    }
}

var client = new RestClient();
client.Proxy = SimpleWebProxy.Default;