Fastest C# Byte[] to String conversion?

Hi, I want to convert byte[] to String to be saved into iCloud Key-Value storage. I have successfully convert the data, but the process is too slow. The data is a ghosting data from several levels which has elements up to 40k. The size of the file is not big, around 100kb. But It could take 2 minutes to finish.

Here is the code I used :

Byte[] to String :

  string ByteArrayToString(byte[] val)
  {
     string b = "";
     int len = val.Length;

     for(int i = 0; i < len; i++) {
        if(i != 0) {
           b += ",";
        }            
        b += val[i].ToString();
     }

     return b;
  }