It is based on an algorithm published on pouet in 2021. One its implementation uses 16 color mode and another uses 4 color mode and textures to simulate 8 colors. Generated pictures are 128x256 and 256x256 raster images respectively. It is very likely that this algoritm implementation is the fastest known Mandelbrot set computation routine for the Z80.
A zipped dsk-image is here: RUN"M4 or RUN"M16 to get 4 (8) or 16 colors.
Sources are available on github.
There is also a nice video recorded from two real CPC6128 on youtube.