I don't have the sources, but emacs can never keep up when I press PgDn or PgUp in any file in any mode. This always happened to me with emacs, maybe since the version that was released say a year ago for windows; I dont remember what version exactly. I can confirm this bug happens in fundamental mode, with emacs -Q, with a large text file that has no large lines.
I have an Acer Aspire One AOD257.
Intel(R) Atom(TM) CPU N570 @ 1.66GHz, 1666 Mhz
Not sure about the display.
Oh! I dont know how to see the exact number, but the keyboard repeat rate was set to max. I lowered it down and it solved the problem. Is it still a bug?