I recently discovered
libjpeg-turbo, a
drop-in replacement for the venerable libjpeg.so.62 that
uses SIMD instructions to achieve 2–4x the performance of the old
library.
I downloaded, built, and installed it (putting it first in
LD_LIBRARY_PATH), and was pleasantly surprised to find it working
exactly as advertised. I have conducted only a few informal tests on
large images, but it's clearly a great deal faster.