MobileSheets caches up to 9 pages in memory, but depending on the file, the speed to render each page can differ greatly between PDFs. If it's a well-optimized PDF, it should load nearly instantly, but I've seen some PDFs that are poorly constructed that can take several seconds per page. If the PDF is rebuilt with a tool like Adobe Acrobat, then it will load immediately. If you use the image correction feature on the PDF, that will rebuild it and may fix your load times (you can find this on the files tab of the song editor). You can also try going to Settings->Display Settings and switching the "Render Preference" setting to see if that has an impact, as it will switch which PDF library is used.
Mike
Mike