Unity Loading Animation

Hello, I was reading lots of post of loading animations with loadLevelAsync etc. But I could not find a solution for my problem:

So at void Start() I create an entire Level String (it goes through a while loop). After that I create an Asset Pool so placing deactivated prefabs on stage (approx. 100 sprite assets).

The entire code takes about 2 sec to be done. I tought, okay I call that create function as a coroutine and Unity wont freeze. That worked just fine but the same calculation took 97 seconds. I placed a yield return in the while loop:

  while (iCurrentPosition < iLevelLength) {
 
 float currentPositionPercentage = 1.0f - (iCurrentPosition / iLevelLengthInPix);
             int iCurrentMin = (int)(iCurrentPosition + ((p_iDifficulty [1] - p_iDifficulty [0]) * currentPositionPercentage) + p_iDifficulty [0]);
             int iCurrentMax = (int)(iCurrentPosition + ((p_iDifficulty [3] - p_iDifficulty [2]) * currentPositionPercentage) + p_iDifficulty [2]);
         
 
                 newDistance = Random.Range (iCurrentMin, iCurrentMax);
                 iCurrentPosition = newDistance;
                 
                 if (amountOfBalks > 0 )
                     m_strLevelString += "&";
                 m_strLevelString += newDistance.ToString() + "-" + getBalkString();
     
                 amountOfBalks++;
                 yield return "";
             }

So I guess after yield it saves the position and continuous after the next frame. That would explain why it takes over 97 seconds.

What I want is, that Unity shows an UI animation without freezing, and removes it after all sceneloading and calculation from start is done and scene is ready to play. How can I solve that?`

Thank you very much for your help! Best regards, Luke