I've been using epeg for an NAS device as it can make thumbnails rather quick (faster than convert). Though it's result might not look as sharp as the result from imagemagick, I think it can be used as an intermediate format when converting large jpegs. If you want to test out epeg, we are keeping a debian package at http://update.excito.org/pool/main/e/epeg/

Following are testing output I've made on my desktop computer (images are attached, original can be found on commons):

[0:0][azatoth@azabox img]$ time epeg Boucher,\ Francois\ -\ Landscape\ Near\ Beauvais.jpg epeg_direct.jpg -w 800 -w 600 -m 800 -q 100

real 0m3.957s user 0m3.460s sys 0m0.076s [0:0][azatoth@azabox img]$ time epeg Boucher,\ Francois\ -\ Landscape\ Near\ Beauvais.jpg epeg_tmp.jpg -w 1600 -w 1200 -m 1600 -q 100

real 0m5.896s user 0m5.120s sys 0m0.088s [0:0][azatoth@azabox img]$ time convert epeg_tmp.jpg -quality 80 -thumbnail 800x -depth 8 epeg_convert.jpg

real 0m0.942s user 0m0.696s sys 0m0.032s [0:0][azatoth@azabox img]$ time convert Boucher,\ Francois\ -\ Landscape\ Near\ Beauvais.jpg -quality 80 -thumbnail 800x -depth 8 convert_direct.jpg

real 0m12.007s user 0m9.929s sys 0m0.800s [0:0][azatoth@azabox img]$ ll Boucher,\ Francois\ -\ Landscape\ Near\ Beauvais.jpg -rw-r--r-- 1 azatoth azatoth 104554102 8 aug 04.16 Boucher, Francois - Landscape Near Beauvais.jpg [0:0][azatoth@azabox img]$ identify Boucher,\ Francois\ -\ Landscape\ Near\ Beauvais.jpg Boucher, Francois - Landscape Near Beauvais.jpg JPEG 12384x10064 12384x10064+0+0 8-bit DirectClass 99.71mb [0:0][azatoth@azabox img]$