An FPS counter.

using UnityEngine;
using System.Collections;

// An FPS counter.
// It calculates frames/second over each updateInterval,
// so the display does not keep changing wildly.
public class Test : MonoBehaviour {
    public float updateInterval = 0.5F;
    private double lastInterval;
    private int frames = 0;
    private float fps;
    void Start() {
        lastInterval = Time.realtimeSinceStartup;
        frames = 0;
    }
    void OnGUI() {
        GUILayout.Label("fps:" + fps.ToString("f2"));
    }
    void Update() {
        ++frames;
        float timeNow = Time.realtimeSinceStartup;
        if (timeNow > lastInterval + updateInterval) {
            fps = (float)(frames / (timeNow - lastInterval));
            frames = 0;
            lastInterval = timeNow;
        }
    }
}