You need to have an xspeed variable, and move it once per frame, and then decrease the value of xspeed.

e.g.
Code:

//in draw()
xpos=xpos+xspeed;
xspeed=xspeed*0.75;
 

Unfortunately you're misunderstanding a major thing about processing.

Nothing gets drawn to the screen until the draw function finishes

So doing xpos+=5;xpos+=4;xpos+=2;xpos+=1 is exactly the same as xpos+=12; since there's no magic that says "oh the position has changed, I'd better update the screen", it just wats until draw has finished, and then things are drawn.

So you need to make it move 5, wait fo rthe next time around draw, move it 4, wait for the next time round, move it 2, wait for the next time round. etc etc.

So you need to do something like:

Code:

int xspeed;
int xpos;

void setup()
{
  //normal setup stuff...
  xpos=width/2;
  xspeed=0;
}

void draw()
{
  //other draw stuff..
  xpos+=xspeed;
  if(xspeed>0) 
    xspeed=xspeed-1;
  if(xspeed<0)
    xspeed=xspeed+1;
  //more drawing stuff maybe?
}

void keyPressed()
{
  if(key==LEFT) 
    xspeed=5;
  if(key==RIGHT)
   xspeed=-5;
} 

No, I'm not, that's what that code does. You press a key once, and then one frame it moves 5, then the next frame it moves 4, the next it moves 3 etc.

Here, this is a cimplete example showing it working, just cut and paste this as a new sketch and you'll see:
Code:

int xspeed;
int xpos;
 
void setup()
{
  size(300,300);
  xpos=width/2;
  xspeed=0;
  frameRate(3);
}
 
void draw()
{
  background(0);
  xpos+=xspeed;
  ellipse(xpos,height/2,5,5);
  if(xspeed>0)  
    xspeed=xspeed-1;
  if(xspeed<0)
    xspeed=xspeed+1;
}
 
void keyPressed()
{
  if(keyCode==LEFT)  
    xspeed=-5;
  if(keyCode==RIGHT)
   xspeed=5;
}