When shrinking a jpg you can get more then double the performance if you use <?php $image->setOption('jpeg:size', '800x532') ?>, exchanging 800x532 to the resolution you want the final image to be. For instance instead of this:
<?php
$image = new Imagick('foo.jpg');
?>
You would use this:
<?php
$image = new Imagick();
$image->setOption('jpeg:size', '800x532');
$image->readImage('foo.jpg');
?>