Jpegview is very slow when I view images of 3240x4320 (14.1 MP).
Also rotating is very slow.
That depends on your hardware, maybe you need an upgrade ;-)
On my Core 2 with 2.66 Ghz and 2 GB RAM (what is really not ultra high speed hardware) pictures of 15 MP open and rotate in a fraction of a second.
So you either have a very slow CPU or not enough RAM.
As I noticed, JPEGView shows JPEG files in portrait orientation slower than landscape ones (without auto rotating by EXIF, of course). What's the reason of such behavior? Is it possible to speed up showing of portrait pics?
I have replace IJL by Turbo JPEG in the next release so maybe it's different with that.
However I do not see any slower loading or processing of portrait images with Canon G10 pictures (14 Mega Pixels).
It's even somewhat faster (not much, but measurable) than landscape.
However I have found that images processed by JPEGTran are considerably slower in loading. No idea why this is the case, I neither know what JPEGTran internally does nor how IJL or Turbo JPEG are decoding the JPEG.
Test method is simple: open a folder with a lot of JPEGs with different orientations, then open first file and press (and hold it on) a key to show next image and look at the speed of picture changes. :)