Thanks for the feedback! I've opened an issue to track this [0]
* Levels 1-19 are the "standard" compression levels.
* Levels 20-22 are the "ultra" levels which require --ultra to use on the CLI. They allocate a lot of memory and are very slow.
* Level 0 is the default compression level, which is 3.
* Levels < 0 are the "fast" compression levels. They achieve speed by turning off Huffman compression, and by "accelerating" compression by a factor. Level -1 has acceleration factor 1, -2 has acceleration factor 2, and so on. So the minimum supported negative compression level is -131072, since the maximum acceleration factor is our block size. But in practice, I wouldn't think a negative level lower than -10 or -20 would be all that useful.