Yeah with such large PDF's it can take some minutes. That seems like forever if you are waiting for it and the reader doesn't respond to any buttons any more at that time.
I would say when you won't need the reader, bring up the PDF and come back to the reader 15 minutes later. Once displayed it will come up much faster.
Also be careful not to touch the magnifier in such a PDF. Then it will reflow the whole PDF and that is what takes the processing time.