Most Efficient Way of Combining Large Strings for HTML Response

Projects Programming
6

As to avoid confusion as much as possible and to get to the root of what I'm hoping to see is this. Let me start with a very simple snippet of code, and if you are willing, provide edits to it.

This is essentially how I have it today. How would you change it?

string returnHTML()
{

     String strHTML = '';
      
      int j =0;
      
      while (j < 20000) {
        strHTML += millis() + ": " + j.toString() + "This is a test.";
        j++;
      }

      return strHTML;
}