MasterServer.RegisterHost causing a bad frame rate?

I am using unity networking for a network game. I am searching for a few hours for a reason for bad FPS and couldn't find anything until I changed something that looks completely unrelated. I run my server with the following lines using the default unity master server:

 Network.InitializeServer(8, mPort, true);
 MasterServer.RegisterHost(mGameName, mServerName, "");

10 seconds after calling this my FPS go down to a level that makes playing impossible. Removing the RegisterHost command fixes the problem. To test it I used an empty scene so there is not much that could cause this. Still if I use RegisterHost the FPS go down. What could be the reason for that? Maybe a bug in unity? Or maybe someone evil on the public master server tries to cause troubles? Has anyone a private master server and can test this?

I use a scene only containing this script on a gameobject + camera:

 using UnityEngine;
 using System.Collections;

 public class FrameRateTest : MonoBehaviour 
 {
     float mTimeSum = 0;


     float mAvgFrameTime = 0;
     float mFps = 0;

     // Use this for initialization
     void Start () {

         Network.InitializeServer(8, 54443, true);
         MasterServer.RegisterHost("testgame1234", "testserver1234", "");
     }
 
     // Update is called once per frame
     void Update ()
     {
         mTimeSum += Time.deltaTime;

         if (Time.frameCount % 60 == 0)
         {


             mAvgFrameTime = (mTimeSum / 60);
             mFps = 1.0f / mAvgFrameTime;

             mTimeSum = 0;
         }
     }


     private void OnGUI()
     {
         GUILayout.BeginVertical();
     
         GUILayout.Space(100);

         GUILayout.Label(mAvgFrameTime + " avg frame time");
         GUILayout.Label(mFps + "fps");

         GUILayout.EndVertical();
     }
 }