GPAINT vs GFILL

So here's something I've discovered when programming the light system for Wonderflonium. Pretty much, don't fill your entire screen with one color using GPAINT; use GFILL instead. To make the light system work, I am just making the screen dark and lighting up areas around light sources. To make the screen totally black, I used GPAINT at first. But this dropped the speed all the way down to an inconsistent 50 FPS, which caused all sorts of bad things like a BG flicker. But when I switched to GFILL, the program ran at a smooth 60 FPS, which looked so much better (especially since I have weapons animated with SPANIM.) I believe this is because GPAINT uses some sort of algorithm to find pixels of the same color, while GFILL only fills the specified area and doesn't have to check if it's allowed to fill certain pixels.