FPSCounterIMGUI.cs

using System.Collections.Generic;
using System.Linq;
using UnityEngine;

public class FPSCounterIMGUI : MonoBehaviour
{
    [SerializeField] int maxFrames = 100;

    Queue<float> avgFPS = new Queue<float>();

    void Update()
    {
        avgFPS.Enqueue(1f / Time.deltaTime);
        if (avgFPS.Count > maxFrames) avgFPS.Dequeue();
    }

    void OnGUI() 
    {
        Rect rect = new Rect(Vector2.zero, new Vector2(100, 25));
        GUI.Label(rect, $"FPS: {avgFPS.Average().ToString("#.##")}", GUI.skin.box);
    }
}