5.0.2 beta
Although our BC7 compression is 100 times faster than Visual Studio's or any other routine I have ever seen, it still takes some time to compress large textures. Since most people don't know or care about the difference between BC7 and DXT5, in this build the editor will default to using DXT5 compression for most textures. This will give you faster texture import, but DXT5 format can display compression artifacts, so it isn't as high quality as BC7. I think for most people this won't matter, and they will just prefer for it to be faster, especially when converting large batches of files.
You can change this setting in the Tools > Options > Material Generation > Compression format setting if you still want to use the slower BC7 compression routine.
Both formats take the same storage space, and I don't think there is any difference in rendering performance.