I somehow completely failed to mention the whole $uses/ClassRegistry/loadModel/chained models thing in my 8 Ways to Speed Up CakePHP Apps post. That's like doing a post on the top 5 adult film stars who hit rock bottom, had it video taped and posted on YouTube, and failing to mention the epic Chasey Lain video (part 2). If you think that link is even remotely SFW, go re-read the description again.

Don't Use $uses Unless You Really, Absolutely Have To

$uses is a controller attribute that allows you to access additional models to the default one. Say you have a blog application and one of the controllers is posts. By default you have access to the Post model. If you wanted to also have access to the Comment model you could do (but shouldn't):

<?php
class PostsController extends AppController {
  var $name = 'Posts';
  var $uses = array('Post', 'Comment');
}
?>

Model Chains

Since Comment is associated to Post through a HasMany relationship you can access the Comment model through Post. Like this:

$comments = $this->Post->Comment->findAllByPostId($id);

The relation chain extends infinitely, including all models down the line.

Controller::loadModel and ClassRegistry::init

Great, but sometimes you do legitimately need access to a model that isn't anywhere in the relation chain. If you are going to use the model throughout the controller go ahead and include it in $uses, but if you only need it in one action there are better ways. They are Controller::loadModel() and ClassRegistry::init().

//the loadModel way
$this->loadModel('Comment');
$comments = $this->Comment->findAllByPostId($id);

//the ClassRegistry way
$Comment = ClassRegistry::init('Comment');
$comments = $Comment->findAllByPostId($id);

Controller::loadModel, not to be mistaken for the deprecated loadModel function, creates an instance of the model and assigns it to the controller. You can then access it the same way as you would as if it was loaded through $uses. ClassRegistry returns an instance of the model. Gwoo prefers the loadModel approach over ClassRegistry. Good enough for me.

Approximate Increase

The dreaded: "it depends." Having one or two extra models in your controller's $uses probably isn't going to kill your app. I added one extra model to my test app and there was about a 4% increase in how long the page took. Then I added seven extra models and there was approximately a 40% increase. So roughly 4-6% for every additional model.