It's currently counting it pixel by pixel and skipping every other pixel. My latest version is doing the same, though it's now skipping over 2 pixels in each direction, so i could reduce the number of pixels checked each frame from 480 to 120 without increasing the calculating time too much. This seems to have reduced the CPU and GPU loads per frame significantly.
I considered downsampling thanks to your suggestion. Unfortunately, it seems gamemaker doesn't have a good way to downsample surfaces outside of, like, redrawing the surface onto another smaller surface, so i figured i could accomplish mostly the same thing by just skipping pixels. The way i went could theoretically lead to cases where some crucial line of ink is skipped that downsampling could have avoided, but i find it unlikely to occur... Downsampling would probably be the better, more accurate, and probably more scalable approach anyways though, so i might try it in the future. Thanks again for suggesting it!