Unity time counter 50% slower FPS?

Hello, I have made a time counter to my game, to show people how long time they have played the game. When I use the script on my PC it work's fine and I don't notice any FPS slowdown. But when I play it on my Android device (Galaxy SIII), it switches from 60 FPS (without the time-script) to 20-30 FPS (with the time-script). Can someone please tell me what's wrong? Here's my script :)

Thanks, Andreas

 using UnityEngine;
 using System.Collections;
 
 public class TimePlayed : MonoBehaviour {
     
     float SecondsPlayed;
     float MinutesPlayed;
     float HoursPlayed;
     
     string Minutes;
     string Seconds;
     string Hours;
     
     int RoundedSeconds;
     int RoundedMinutes;
     int RoundedHours;
     
     
     public GUIText SecondsPlayedDisplay;
     
     
     
     // Use this for initialization
     void Start () {        
     SecondsPlayed = PlayerPrefs.GetFloat("SecondsPlayed", SecondsPlayed);
     MinutesPlayed = PlayerPrefs.GetFloat("MinutesPlayed", MinutesPlayed);
     HoursPlayed = PlayerPrefs.GetFloat("HoursPlayed", HoursPlayed);
     }
     
     // Update is called once per frame
     void Update () {
     
         RoundedSeconds = Mathf.RoundToInt(SecondsPlayed);
         RoundedMinutes = Mathf.RoundToInt(MinutesPlayed);
         RoundedHours = Mathf.RoundToInt(HoursPlayed);
         
         
     //Second or Seconds
     if (RoundedSeconds == 1) {
         Seconds = "Second";    
     }
         
     if (RoundedSeconds != 1) {
         Seconds = "Seconds";        
     }    
         
         
     //Minute or Minutes
     if (RoundedMinutes == 1) {
         Minutes = "Minute";    
     }
         
     if (RoundedMinutes != 1) {
         Minutes = "Minutes";        
     }    
         
     //Hour or Hours
     if (RoundedHours == 1) {
         Hours = "Hour";    
     }
         
     if (RoundedHours != 1) {
         Hours = "Hours";        
     }        
             
         
         
         
     SecondsPlayed += Time.deltaTime;
         
     if (SecondsPlayed >= 60f) {
     SecondsPlayed = 0f;
     MinutesPlayed += 1f;
     }
         
     if (MinutesPlayed >= 60f) {
     MinutesPlayed = 0f;
     HoursPlayed += 1f;
     }    
         
     SecondsPlayedDisplay.text = (Mathf.RoundToInt(HoursPlayed).ToString() + " " + Hours + ", " + Mathf.RoundToInt(MinutesPlayed).ToString() + " " + Minutes + " and " + Mathf.RoundToInt(SecondsPlayed).ToString() + " " + Seconds );
         
         
     PlayerPrefs.SetFloat("SecondsPlayed", SecondsPlayed);
     PlayerPrefs.SetFloat("MinutesPlayed", MinutesPlayed);
     PlayerPrefs.SetFloat("HoursPlayed", HoursPlayed);
      }
 }