If you are coverting your CBR/CBZ comics to a PDF file make sure to resize the images to the resolution of the screen this will ensure that the page turning is very fast. If the iPDF viewer has to rescale the images it might be a bit on the slow side.
If you are using the full screen ipdf version developed by the wiz kids here, which is great for viewings comics
, than you have to resize the images to 768x1024. If you are using the standard ipdf viewer you will have a toolbar at the bottom and than you best use resolution 768x935.
I hope this helps :-)