ok, now I'm confused, I find out how to decrease the size of visual feedback, but is it the same as area? I thought that if area will be smaller than it eats less CPU.

by the way, whats wrong with top and bottom line?

1. void setup() {
2.   size (500, 400);
3.   background (0);
4.   rectMode (CENTER);
5. }
6. 
   
7. void draw() {
8.   background(0);
9.   for (int x = 25; x < width; x +=50) {
10.     for (int y = 25; y < height; y +=50) {
11.       float d = dist (x, y, mouseX, mouseY);
12.       fill (255-d, 0, 0);
13.       float s = map( d, 0, 255, 50, 0 );
14.       rect (x, y, s, s);
15.     }
16.   }
17. }

You were setting the fill color after drawing the rectangle. That was causing the bottom/top issue. Your sketch was a bit too big to run smoothly, so I shrank it a bit too. And rectangles now resize based on distance to mouse.