The following simple program:
namespace Proto1.Client
{
using Akka.Actor;
using Akka.IO;
using System;

class Program
{
    static void Main(string[] args)
    {
        var system = ActorSystem.Create("proto1");
        var manager = system.Tcp(); //this causes 100% CPU
        Console.ReadLine();
    }
}

}

On my machine it uses around 25% CPU. Starting the console app 4 times gives me 100% CPU utilization:

I am using i7 cpu:

Using nighly build Akka 1.0.4.64-beta