slow blitting

hi , i have a weird problem.

im making a isometric tile engine.
and in 1024*768 pixels everything runs extremely smooth.

but when i change the res to 800*640 everything goes slowmo'

i cant figureout why.
coz i draw the result to a picturebox which is the same size both in 800x640 as in 1024*768

so both the picturebox and my backbuffer has the same size in both resulotions.

the backbuffer and the gfx buffers that holds the tiles / units / transparensy masks is created using "createcompatibledc" and "createcomptiblebitmap"

any ide'as what can be wrong???

info:
platform: win2k
cpu : amdk6 2
gfxcard : ati rage lt pro.


could it be the litlle ati card?
and if so , what should i do to solve it.
i cant tell everyone who want to try the engine to buy a new gfx card...


/Roger