It is based on an algorithm published on pouet in 2021. It uses 16 colors. Generated pictures are 256x128 raster images. It is very likely that this algoritm implementation is the fastest known Mandelbrot set computation routine for the 65816. However, the plot is made rotated because the IIgs is not capable to show 256 raster lines, and the rotated plot is slower than the normal.
A zipped bootable dsk-image is here. As an extra it contains a fullscreen (320x200 16 colors) Mandelbrot generator which can draw, for instance the next image.
Sources are available on github.