I always had the notion that LLVM is faster than GCC. I don't feel qualified to interpret this benchmark comparison, but they appear to have roughly the same performance?
LLVM compiles faster than GCC (although the gap has narrowed in the last couple of years). GCC produces faster code than LLVM (although that gap has also narrowed)
This recent presentation from the 2013 European LLVM Conference outlines the optimization issues LLVM still suffers from, making the produced binaries up to ~10% slower than with GCC for some use cases: