GraphicsMagick is a fork of ImageMagick which has the exact same command line but can be significantly faster for some operations. http://www.graphicsmagick.org/benchmarks.html

You simply prepend "gm" to the command line.

gm convert ...
gm mogrify ...
gm identify ...

I was thinking you could just have a flag to set which enables it, like so:

MiniMagick.use_graphicsmagick = true

and update the run_command method to use it.