Dimitry Andric, a FreeBSD developer, has carried out some performance tests to explore the impact that LLVM/Clang as the default FreeBSD compiler has on FreeBSD 10, compared to GCC 4.2.1 and GCC 4.7.1. He concludes that  to build FreeBSD with Clang less RAM is used and the compilation finishes quicker. Clang comes out in the benchmarks mostly ahead of GCC on FreeBSD.

I recently performed a series of compiler performance tests on FreeBSD 10.0-CURRENT, particularly comparing gcc 4.2.1 and gcc 4.7.1 against clang 3.1 and clang 3.2.

The attached text file[1] contains more information about the tests,
some semi-cooked performance data, and my conclusions. Any errors and omissions are also my fault, so if you notice them, please let me know.

The executive summary: clang compiles mostly faster than gcc sometimes much faster), and uses significantly less memory.

Finally, please note these tests were purely about compilation speed,
not about the performance of the resulting executables. This still
needs to be tested.

You can check the benchmarks here: Clang/llvm performance tests on FreeBSD 10.0-CURRENT