dan@Cairo:~$ time convert -resize 640x480 nasa_image.jpg nasa_image-im.jpg

real    0m10.416s
user    0m8.279s
sys     0m0.622s

dan@Cairo:~$ time sips -Z 640 nasa_image.jpg --out nasa_image-sips.jpg
/Users/dan/nasa_image.jpg
  /Users/dan/nasa_image-sips.jpg

real    0m1.537s
user    0m0.950s
sys     0m0.090s