I've enjoyed working on the Zoop Framework's Lunar branch, and it's gotten pretty rad in the last several months. I wanted to compare it with some of the other offerings.
For some background, watch Simple is Hard, Rasmus Lerdorf's really long talk at Drupalcon Szeged last year. Make sure to check out the presentation slides for all the gory details. If you just want to see some numbers and pretty charts, see below.
I wrote a quick Hello World in the latest versions of CakePHP, CodeIgniter and Symfony, and compared them with the a skeleton app from Zoop's Lunar branch. I kept each of the apps as vanilla and basic as I could, so they look a lot like the ones Rasmus wrote.
For comparison, I also included a trivial PHP and HTML page with the exact same output
<html>
<head>
<title>Test</title>
</head>
<body>
<h1>Hello world!</h1>
</body>
</html>
The output looks like just like that for each of the frameworks.
Each framework was responsible for duplicating the following PHP snippet
<html>
<head>
<title><?php echo 'Title'; ?></title>
</head>
<body>
<h1><?php echo 'Hello world!'; ?></h1>
</body>
</html>
Here are the good parts of the Zoop app
class zone_default extends Zone {
function pageIndex() {
global $gui;
$gui->assign('title', 'Test');
$gui->assign('message', 'Hello world!');
$gui->display('base/hello_world.tpl');
}
}
<html>
<head>
<title>{$title}</title>
</head>
<body>
<h1>{$message}</h1>
</body>
</html>
This kept the differences to a minimum. All of my tests were done on a virtual machine with PHP 5.2.4, and tested with 15 concurrent connections, using Siege in benchmark mode. These benchmarks essentially measure the minimum overhead of each framework while completing a trivial task. Note that it doesn't address feature sets, functionality, ease of use, available libraries and support, or any of the other reasons you might want to use a framework... That's another blog post for another day.
Here's the big picture

The HTML and trivial PHP pages are obviously fast. And APC (the blue sections) is worth every cent. It's a pretty chart, too. Raw PHP is twice as fast as CodeIgniter, which is twice as fast as Lunar. Lunar is twice as fast as Symfony, and CakePHP just crawls, even with APC enabled. Scroll to the bottom of this post to see all the numbers, if you're into that sort of thing.
Tweaking Zoop for even better performance after the jump.