https://docs.microsoft.com/en-us/dotnet/api/system.net.networkinformation.networkinterface.speed?view=netframework-4.8
- NetworkInterface.Speed Property

Namespace: System.Net.NetworkInformation

public static void ShowInterfaceSpeedAndQueue()
{
    NetworkInterface[] adapters = NetworkInterface.GetAllNetworkInterfaces();
    foreach (NetworkInterface adapter in adapters)
    {
        IPInterfaceProperties properties = adapter.GetIPProperties();
        IPv4InterfaceStatistics stats = adapter.GetIPv4Statistics();
         Console.WriteLine(adapter.Description);
        Console.WriteLine("     Speed .................................: {0}", 
            adapter.Speed);
        Console.WriteLine("     Output queue length....................: {0}", 
            stats.OutputQueueLength);
    }
}