The ETC1S encoder supports multiple compression "effort" levels using the -comp_level X command line option, where X ranges from [0,5]. Note the -q X option is the main option to control the quality level of ETC1S .basis/KTX2 files. -comp_level is mostly intended for harder to handle content such as texture video. It modifies a number of internal encoder configuration parameters, which slows it down a bunch but allows it to achieve slightly higher quality per output bit. For texture video, higher comp levels can significantly reduce artifacts.

This option (along with -q or manually setting the codebook sizes) controls the tradeoff between ETC1S encoding time and overall quality. The default is level 1, which is the sweet spot between encoding speed vs. overall quality. Here's a graph showing the encoding time and average quality across 59 images for each level:

This benchmark was done on a 20 core Xeon workstation. The results will be different on less powerful machines.

Level 0 is fast, but this level disables several backend optimizations so it will generate larger files. It will also be quite brittle on complex textures or artificial textures.

Note that -comp_level 3 is equivalent to the initial release's default, and -comp_level 5 is equivalent to the initial release's "-slower" option. Also, -slower is now equivalent to level 2 (not 4).