No, gcc is the slowest compiler, AFAIK. Its advantage is being very portable across architectures, which isn't true for Intel's or ARM's.
And what's about the (recently re-opensources) Pathsscale? It is on par with Intel, in my experience. And then there is the PGI.... The compiler's game is a large race and GCC and LLVM are not the only two horses in it.
I believe Intel does its own closed source compiler to show the best it can on its hardware. They don't want that "best" being available on other vendor's hardware.