I haven't used clang much in recent years but i remember back when it was first introduced clang was faster than gcc while producing only slightly slower (or sometimes comparable) code.

Most likely compile times slowed down as clang and llvm became more complex, but early clang was faster enough for people to switch to it - and then just stayed with it (this isn't a unique case, people switched to Chrome from Firefox because Chrome was much faster and they stayed with Chrome even after Chrome became slower and Firefox faster).

In any case the comparison was with gcc (and perhaps msvc), not all types of compilers.