should generate an image example.png in the current directory.
Note that the image generation does not take advantage of GPUs or
-multi-core, and can be as slow as 40 vignettes per second s on a 4GHz
-i7-6700K.
+multi-core, and can be as fast as 3,000 vignettes per second and as
+slow as 40 on a 4GHz i7-6700K.