I have been investigating the same question myself. Here is a benchmark of different software to do image resizing:
http://www.vips.ecs.soton.ac.uk/index.php?title=Speed_and_Memory_Use
Personally I would suggest that you use the https://github.com/aheckmann/gm library, because it runs approximately 1.5-5 times faster than imagemagick according to this:
http://www.admon.org/graphicsmagick-vs-imagemagick/ (cant post more than two links. reputation)
Good luck, I went for graphicsmagick myself.