Apple knows this is not a good solution, so a compromise is that if the page is not zoomed (i.e. zoom = 1.0), then the page will draw in one pass. This is good for any PDF viewer that displays non-zoomed pages - like iBooks - notice if you zoom, then turn a page, it goes back to 1.0 zoom.
With version 1.6 of unrealBook, new PDFs that are added will draw in one pass if zoom is 1.0 (just double-tap the center of the screen). Otherwise if you zoom the document it will perform a dual pass update of the document when turning pages. Version 1.6 tries to speed this up by rendering pages ahead of time both before and after the current page. Notice that only new PDFs will draw like this, due to a technical mistake (i.e. bug etc), older files will always draw in two passes.
What if you want old PDFs to draw in one pass with zoom 1.0? There are 2 ways to achieve this. Go to Settings->File and choose Convert to New, or if you don't have any drawings or hot spots etc... choose Remove all support files. The only advantage you gain is that the page will draw in one pass, however you will probably want to zoom the document which will make the zoom not be 1.0 and then it will still perform the 2 pass drawing.
New PDFs added to unrealBook under iOS5 will draw in one pass if zoom is 1.0. If you wish to view a larger full screen image - you need to adjust the cropbox of the PDF document using a PDF editor or something like Preview for the Macintosh. Go to Tools->Select tool, then drag over the part of the document you want to be full screen, then choose Crop from the Tools menu.
Any old document or any document that is zoomed will have a preview image (slightly blurry) before a higher resolution image drawn.
Suggestions have been sent to Apple to make iOS5 update like iOS4 did.