Why? the Phi uses customized Atom cores but a lot of them, it's not designed to execute normal x86 code particularly fast.
It's designed to be 100% code compatible, you can boot windows/linux on it and run your programs with the specialized SIMD instructions for actual throughput.
You get code compatibility, cache and branch management from x86 CPUs and near GPU speeds (or even faster for FP64) for SIMD instructions.
The cores are individually slow and non-parallel code will not be fast, but the Airmont architecture it's based on is an out-of-order core, so it's not that bad as earlier versions of the Phi, plus it has up to 16GB of very fast HMC memory near the cores, helping make up for the slow cores with less costly cache misses than an ordinary Xeon. It also has four threads per core without the penalties of earlier Phis. Single-thread stages will suffer but that may end up being compensated by ridiculously parallel compilation stages.
But yes. The Phi is designed to shine with code that uses its AVX512 units. Compiling the Linux kernel is a "create misuse" of its technology. One I'd like to try one day ;-)
It's designed to be 100% code compatible, you can boot windows/linux on it and run your programs with the specialized SIMD instructions for actual throughput.
You get code compatibility, cache and branch management from x86 CPUs and near GPU speeds (or even faster for FP64) for SIMD instructions.