stb_dxt v1.09, icbc, rgbcx v1.12, original crunch, and Unity's optimized variant of crunch. Both 4 and 3 color blocks can be used, but transparent texels are not utilized to get black/dark texels in this benchmark. Across a diverse assortment of 100 textures (not just images).
Same benchmark except this time with 3-color transparent texels used for black or dark texels in rgbcx (purple samples):
Here's an update, now with nvdxt.exe (black sample) and ispc_texcomp (brown sample). Note that the nvdxt.exe time is approximate because I had to spawn nvdxt.exe and it loads a .png and saves a .dds file. I did spawn it twice, once without timing it, then immediately again timing it.
nvdxt.exe command line:
nvdxt.exe -nomipmap -quality_highest -rms_threshold 50 -file image.png -output nvcompressed.dds -dxt1c -weight 1.0 1.0 1.0