Are you converting multiple images at the same time? If you convert too much images at the same time you can run out of memory and Magick.NET will switch to disk cache which is terribly slow.
Have you measured which part of your action is slow? Is it reading/writing of the image or is it the resize action? And what do you consider slow?
Have you measured which part of your action is slow? Is it reading/writing of the image or is it the resize action? And what do you consider slow?