On the BitFlux blog today, Christian has posted a very brief look at some benchmarks between GD and imagemagick.

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.

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.

If you'd like to run them yourself, you can check out the benchmark script he used...