Can you describe what you mean by "the program does not respond well"?
I've worked with datasets of more than 10.000 lines in the past. There was no problem using loadStrings().
It might help to see the code you have so far, i don't think that loading the text-file should make problems.

If for example you use the following code with a large text performance drops significantly.

You try to display hundred or thousand of lines, while only a small part fits on screen.
If you restrict the text() call to the parts that are actually displayed, it should be much faster.

Alright, so PhiLho you recommend to set from the for loop structure a specific number of array items that are activated (i.e. only 100 of them) at a particular moment, and change these 100 items again and again based on the Up/Down navigation?

I think what he means is that it is not necesssary to draw all the 15.000 lines, if your stage only has room for about 30 of them. Just calculate, which of them will be visible, and only draw these.

Example:

1. int textY = 10;
   int textX = 10;
   int start = 0;
   int numLines;
   int yDist = 15;
   
   String[] lines;
   
   void setup() {
     size(500, 400);
     lines = loadStrings("data.txt");
     numLines = height/yDist;
   }
   
   void draw() {
     background(0);
     if (keyPressed) {
       if (keyCode==UP) {
         textY = min(0, textY+5);
       }
       else if (keyCode==DOWN) {
         textY = max(textY-5, -(lines.length-numLines)*yDist);
       }
     }
     start = abs(textY/yDist);
     for (int i = 0; i < numLines; i++ ) {
       text(lines[i+start], textX, (yDist * i), width, 20);
     }
   }
   

Incredible, this works fine!

println(numLine) after its setting in setup(): you will see you display only few lines on screen instead of thousands outside of the screen.
start allows to pick up these lines inside the array of text.