Just a quick note. For a client project, I had to compare the performance of GD, ext/imagick and ext/magickwand for reading and merging two alpha-channel PNG24 images and writing them back to disk. Here's the result (the absolute figures don't mean anything):
magickwand 17.48 imagick 17.57 gd 14.81
gd wins slightly, and the two imagemagick extension show (unsurprisingly) the same performance. And if you merge more than 2 pictures together, gd's lead increases.
Conclusion: We will stay with GD for that particular task.
The source for the benchmark script. If someone has improvement suggestions, I'm all ears .)