1. int ballSize = 15;
2. float ballX, ballY;
3. float speedX = 7;
4. float speedY = 5;
5. int dirX = 1;
6. int dirY = 1;
7. 
   
8. void setup() {
9.   size(900, 600, P3D);
10.   background(200);
11.   smooth();
12.   //noStroke();
13.   //noCursor();
14.   camera(width/2.0, 600, 500,
15.   width/2.0, height/2, -200,
16. 
    
17.   0, 1, 0);
18.   ballX = width/2;
19.   ballY = height/2;
20. }
21. 
    
22. void draw() {
23.   background(200);
24. 
    
25.   pushMatrix();
26.   translate(0, 0, -10);
27.   fill(255);
28.   rect(0, 0, 900, 600);
29.   popMatrix();
30. 
    
31.   ballX += speedX * dirX;
32.   ballY += speedY * dirY;
33.   fill(0);
34.   ellipse(ballX, ballY, ballSize, ballSize);
35. 
    
36.   if (ballX >= width || ballX <= 0)
37.     dirX *= -1;
38. 
    
39.   if (ballY >= height || ballY <= 0)
40.     dirY *= -1;
41. 
    
42.   println(frameRate);
43. }

there's a technique to store fixed scenery (floor, walls...) in your graphic card - could be fast. Look it up here in the forum please.