Best way to change alpha on per object basis. (c#)

I'm creating a GUI pop-up system and each pop-up "flashes" by changing the alpha from 1.0 - 0 - 1.0 over 2 seconds using GUI.color.a, But when 5 of these pop-ups are created it starts to lag the game, Comment out the alpha bit and it doesn't lag the game. I'm doing it like so:

Popup class:

 void Update(){
             
     if(fadeIn){
         if(alpha < 1.0f) { alpha += Time.deltaTime*2; }
         if(alpha > 1f) { fadeIn = false; }
     } else {        
         if(alpha > 0.0f){ alpha -= Time.deltaTime*2; }
         if(alpha < 0f){ fadeIn = true; }    
     }
 }
 
 public float GetAlpha(){
     return Mathf.Clamp01(alpha);
 }

On the popup Overseer:

 void OnGUI(){
     if(popups.Count > 0){
         foreach(GameObject obj in popups){
             
             ScreenPopup popup = obj.GetComponent<ScreenPopup>();
             Vector3 screenPoint;

             // --- other code in here
             
             Color GUIcol = GUI.color;
             GUIcol.a = popup.GetAlpha();
             GUI.color = GUIcol;
             
             GUI.DrawTexture(new Rect(screenPoint.x, ( Screen.height - screenPoint.y),30,30), popup.GetTexture());
         }
     }
 }

Anyone know of a better system for this?