Using setSize before reading an image file tells ImageMagick to resize the image immediately on load - this can give a substantial increase in performance time and save memory and disk resources for large images:

<?php

$image = new Imagick();
$image->setSize(800,600);
$image->readImage($file);

?>

This might also save you having to call thumbnailImage to resize the image. 

On my server, this only made a difference with jpgs - pngs and gifs were loaded at full size which took much longer (30s or more compared to 6s for a similar sized jpg).