Fastest JPEG thumbnail generator for Linux

The fastest library I've used so far is definitely EPEG. It can only create thumbs from JPG files, but it creates them very fast:

$ identify worldmap.jpg
worldmap.jpg JPEG 6400x3200 6400x3200+0+0 DirectClass 8-bit 6.85727mb 0.840u 0:02

Imagemagick:

$ time convert -resize 1536x768 -quality 50 worldmap.jpg im_thumb.jpg
# 2.93s user 0.23s system 85% cpu 3.718 total

EPEG:

$ time epeg -w 1536 -h 768 -q 50 worldmap.jpg epeg_thumb.jpg
# 0.31s user 0.01s system 79% cpu 0.404 total

I used this library to create thumbs for a vast amount of really large images.

I've solved this problem by downloading swiggle (a command-line JPEG thumbnail + HTML image gallery generator), and modifying its source for my needs. See the speed measurements in the question (less than 10% faster than djpeg+pnmscale+cjpeg).