Hmm. Not sure what you can do, but I have some ideas.
If there are background images, it means that the PDF pages have not been "flattened". In other words, the PDF is displayed by first clearing the page to the background color, then drawing the background image on top, then drawing the text on top of that.
It's possible that the image is very high resolution and is being dynamically scaled on-the-fly. That's a very cpu-intensive process and would result in the slowdown you're experiencing.
You say it's password protected, so you probably can't print it. But if you can, you can try printing it to a printer configured for Postscript, but choosing the "Print to File" box when you actually print from inside Acrobat Reader. This will convert the document to Postscript. Then use one of the free services on the web to convert it back to PDF. Or use the full Adobe Acrobat product. When the Postscript is converted back to PDF, most tools let you choose a scaling factor for images so that the image embedded inside the PDF has already been scaled. This should be a huge speedup if the problem is dynamic image scaling. (I would recommend an image scale of 150 dpi or so; that's a reasonable resolution for most printers. At 300 dpi there is four times as much data being manipulated, so smaller is geometrically faster to process.)
There are also hacked versions of tools that can bypass some of the password settings so that you might be able to accomplish the "printer workaround" even if the document is password protected. You'll need to search for those tools yourself.
This is one reason why I prefer watermarked PDFs such as those from Paizo instead of password protected documents. The passwords are not THAT tough to get around and once you have there is no protection for the owner of the PDF. At least with watermarks, there is little or no incentive to try to "workaround" the protection and yet people can still optimize the PDF for their environment...
