libvips can convert PDF -> JPEG quickly. It comes with most linux distributions, it's in homebrew on macos, and you can download a windows binary from the libvips site.
This will render the PDF to a JPG at the default DPI (72):
vips copy somefile.pdf somefile.jpg
You can use the dpi option to set some other rendering resolution, eg.:
vips copy somefile.pdf[dpi=600] somefile.jpg
You can pick out pages like this:
vips copy somefile.pdf[dpi=600,page=12] somefile.jpg
Or render five pages starting from page three like this:
vips copy somefile.pdf[dpi=600,page=3,n=5] somefile.jpg
The docs for pdfload have all the options.
With this benchmark image, I see:
$ /usr/bin/time -f %M:%e convert -density 300 r8.pdf[3] x.jpg
276220:2.17
$ /usr/bin/time -f %M:%e pdftoppm -jpeg -r 300 -f 3 -l 3 r8.pdf x.jpg
91160:1.24
$ /usr/bin/time -f %M:%e vips copy r8.pdf[page=3,dpi=300] x.jpg
149572:0.53
So libvips is about 4x faster and needs half the memory, on this test at least.
