It is based on an algorithm published on pouet in 2021. It generates 512x256 monochrome raster images. Actually only 128x256 Mandelbrot dots are computed and plotted but 4x1 texture bricks are used to simulate 8 colors. It is very likely that this algoritm implementation is the fastest known Mandelbrot set computation routine for the Z80.
An archive with executables is here: just run
M2 under CP/M to get a Mandelbrot gallery.
You can also try
M2FS to get a fullscreen variant (720x256, v4). This variant doesn't use textures, so all 720x256 dots are computed. The next image is generated by this program.
Sources are available on github.