convert image.png -crop 256x256 +repage +adjoin out_%d.png
takes for a large image 20000x8000 aprox 114 seconds. (about 4000 256x256 pngs are generated)
Are there any imagemagick options to improve the speed of cropping?
One thing you can do is be sure to use a "Q8" build of ImageMagick rather than the default "Q16", if your original images have 8-bit samples. Each pixel occupies 8 bytes (16-bit R, G, B, A) even if it's just a black-and-white drawing. Using Q8 cuts that in half. You can change the behavior with the "-limit" option, to use more memory instead of disc.
The -limit option is described in the "options" documentation for ImageMagick.
You can improve the speed of PNG compression by using "-quality 1" which selects Huffman-only compression and the "sub" PNG filter. If you know that the image has fewer than 256 colors, "-quality 0" might be more effective (the time consumed will be about the same but the resulting file will be smaller).