After further investigation into imagemagick (here's the last post to this matter), I realized, that you can improve performance quote a bit, if you set the correct compression quality (in magickwand it's MagickSetImageCompressionQuality($m,$level)). With a quality setting of 30, it was equally fast as GD and the picture was a lot smaller (40.5kB vs. 67kB). If size is really important to you, imagemagick with high settings is almost as good as pngcrush (34'518 vs 34'342, with quality = 90).

imagemagick has a strange understanding of that quality setting. For a longer description, see the manual, but in short, the 10^1 numbers are the compression level, the 10^0 are the filters used. The default setting is 75, that means a compression of 7 and ā€œadaptive filtering is used when quality is greater than 50 and the image does not have a color map, otherwise no filtering is usedā€. But accidently, this was almost the worst compression ratio, the picture came out with 66kB, while not being really fast.

What do we learn from this? Imagemagick (and the php extensions) are very powerfull and the correct settings are very important. If size and/or speed is important to you, it's a good thing to play with those settings and benchmark them with your pictures. GD doesn't have a setting for the compression of PNG images, btw, so you always end up with quite large PNGs compared to imagemagick with correct settings or pngcrush.