Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
Try rebuilding ImageMagick without OpenMP support. There are a few buggy implementations of libgomp that slows down ImageMagick rather than speed it up. We're using ImageMagick 6.5.7-0 and your command completed in 2 seconds.