It sounds very strange that a full-screen GUI will slow down your computer. I have a 2.4 GHz P4 and experience absolutely no slowdown with such an effect. You should try updating your graphics drivers and DirectX.
However, as an alternate solution, have you tried using AGS's built-in fade function?
Code: ags
However, as an alternate solution, have you tried using AGS's built-in fade function?
SetFadeColor(255, 255, 255);
FadeOut(64);
FadeIn(20);
SetFadeColor(0, 0, 0); //to return to default black fades