Saving a few floats won't come anywhere near overloading Unity; something is wrong with the code you wrote. You save the position by writing the x, y, and z coords of the player using PlayerPrefs.SetFloat. If you want to make it a little simpler, you can use ArrayPrefs, which would let you write

PlayerPrefsX.SetVector3 ("PlayerPosition", transform.position);

Assuming transform is the transform of the player. Then you load it by doing this:

transform.position = PlayerPrefsX.GetVector3 ("PlayerPosition");

On some platforms (especially Android and iOS) it can be really slow to save a lot of data in PlayerPrefs. Sometimes it can take multiple seconds, giving the impression that Unity crashed.

This could be the case if you're saving a lot more than just the player position.

A solution is to write a custom PlayerPrefs class where the data is only saved periodically to a file.

We've shared our own PlayerPrefs class for anyone to download: http://www.previewlabs.com/writing-playerprefs-fast/

here is a working script

#pragma strict
var player : Transform;
var x : float;
var y : float;
var z : float;
function Start () {

 if(PlayerPrefs.HasKey("x") && PlayerPrefs.HasKey("y") && PlayerPrefs.HasKey("z")) {

 x = PlayerPrefs.GetFloat("x");
 y = PlayerPrefs.GetFloat("y");
 z = PlayerPrefs.GetFloat("z");

 player.position.x = x;
 player.position.y = y;
 player.position.z = z;

 }
}

function Update () {

  x = player.position.x;
  PlayerPrefs.SetFloat("x", x);

  y = player.position.y;
  PlayerPrefs.SetFloat("y", y);

  z = player.position.z;
  PlayerPrefs.SetFloat("z", z);

}