How to faster pinging addresses using C# Ping

Question

I am constructing a function pinging all the addresses in my network. I have set the timeout be 15 mil seconds, yet each ping takes around 4 to 6 seconds for a reply. Imagine, 4 to 6 seconds loop 254 times. It almost took you half an hour to complete a broadcast ping.

Even worse, when it reaches an address that is occupied by a device, it stucks! the program can't move on anymore. No more debugging message.

I try manually type a for-loop command in cmd for /l %i in (2,1,255) do ping 192.168.137.%i -n 1 -w 15 | find /I "TTL", all the process takes not more than 2 minutes.

*All those things are under a sub-thread, so it will not block the main program.

public IPAddress[] GetAllConnectedAddressInVirtualNetwork () {
    List<IPAddress> ipList = new List<IPAddress> ();
    string networkAddress = GetHotspotDefaultAddress (); //e.g. 192.168.137.1
    string networkPrefix = networkAddress.Substring (0, networkAddress.Length - 1); //e.g. 192.168.137.
    UnityEngine.Debug.Log("Start scanning in "+networkPrefix+"x");
    //Scanning from x.x.x.2 to x.x.x.255
    for (int i = 2; i <= 255; i++) {
        System.Net.NetworkInformation.Ping pingSender = new System.Net.NetworkInformation.Ping ();
        PingReply pingReply = pingSender.Send (networkPrefix + i.ToString (), pingTimeout, Encoding.Default.GetBytes(""));

        if (pingReply.Status == IPStatus.Success) {             
            ipList.Add (pingReply.Address);
            UnityEngine.Debug.Log (pingReply.Address.ToString () + " added to list.");
        } else {
            UnityEngine.Debug.LogError (pingReply.Status.ToString ());
        }
        UnityEngine.Debug.Log ("pinged to "+networkPrefix + i.ToString ());
    }
    return ipList.ToArray();
}