That actually depends. I'm some cases I've seen Clang pull off superhuman feats of vectorization, especially on ARM64. You paste code in Godbolt Compiler Explorer and GCC produces fairly readable assembly, and Clang uses every trick in the book to vectorize. In those cases Clang can be substantially faster. But code needs to be written with vectorization in mind for that to happen (short loops of known size, and a few other restrictions like that). Most of the time it's a few percent behind GCC.