It’s not entirely trivial (if you care about texture quality) you’re choosing 2 endpoints in 3d space that can be linearly interpolated to values that best fit 16 points in 3d space.

I may have been a little off with saying seconds per texture but it’s definitely non trivial amounts of time. And ects (mobile) and BC7 are certainly still not something you want to compress to at game load-time.

Some comparisons of texture compression speeds for various formats (from 2020 but on a 12core i9): https://www.aras-p.info/blog/2020/12/08/Texture-Compression-...