I don't think that it's convincing evidence that clang is slower than GCC.
I was thinking about, and meaning to post that one, but got the wrong search result:
https://www.phoronix.com/review/clang20-gcc15-amd-znver5/5
But they swapped places a few times over the last years, so basically there doesn't seem to be a fundamental difference in performance anymore.
I don't think that it's convincing evidence that clang is slower than GCC.