I don't know what directionoffset stand for…but I think this does the job:

1. float x, y, dx, dy, directionOffset;
   void setup()
   {
     size(500, 500);
   }
   void draw()
   {
     background(200);
     move();
     ellipse(x, y, 20, 20);
   }
   
   
   void move() {
     if (keyPressed==true) {
       if (key==CODED) {
         if (keyCode==UP) {
           dy--;
         }
         else if (keyCode==DOWN) {
           dy++;
         }
         else if (keyCode==LEFT) {
           dx--;
         }
         else if (keyCode==RIGHT) {
           dx++;
         }
       }
     }
     x = x + dx;
     dx = 0;
     y = y + dy;
     dy = 0;
   }
   
   

dirrectionOffset is basically the ++ -- things, it just I needed to increment more than 1px a time so I declared it outside of the method. Its value is   = 3. So it increments each time by 3px instead of 1.

Perhaps it is an issue with your keyboard buffer: the auto-repeat feature of your keyboard might be fast, so you have a few keystrokes stored in the buffer that needs to be processed before the new key can be.
FYI, users reported differences of behavior of keyboard depending on the system (Windows / Linux / Mac).

Is there a way to fix this ? Or to disable the buffer? just go from pure input without storing any previous keys.

maybe use void keyReleased() instead of keyPressed…