I guess that your resolution could be high, maybe - it's surprising how small 1 pixel becomes at higher res. The PixelsPerInch could affect things but I'm not sure if it affects things like you'd expect (I know it affects font sizes and the overall size of the window, at runtime, but does it actually change the pixel sizes in general? Don't think it does but I may stand corrected here).

Does the movement itself seem jerky? Do the graphics seem pretty slow?

Also, watch for moving using the key events (onKeyPress, onKeyDown, or whatever). They may result in not-entirely-smooth movement that's a little stop-start. It may be a better idea to check often whether a key has been pressed at regular intervals and make a note of it, moving then.