I see that you have a single pixel on the bottom left that represents the background. What you're doing is just making Graal's job more difficult because it has to tile a single pixel rather than a larger set, which takes more CPU time.
If you want a solid colored background simply remove the background pixel from the image and set opaque to true in the Window's profile then change fillcolor accordingly.
Attached is the image I'm using for the simple gray window theme based on VGUI in the Source Engine. The image has 80 percent opacity.
And combined with this profile, it makes the window background to what I want it to be without having to put it in the image file.
PHP Code:
new GuiBlueWindowProfile("GuiGrayWindowProfile") {
this.bitmap = "guigray80_window.png";
this.fonttype = "Verdana";
this.fontsize = 14;
this.fontstyle = "b";
this.fontcolor = {255, 255, 255};
this.fontcolorna = {176, 176, 176};
this.fontcolorhl = {0, 0, 0};
this.opaque = true;
this.fillcolor = {88, 88, 88, 255 * 0.80};
this.fillcolorhl = {255, 164, 0};
}
The result is also attached.