The Phi... definitely slower. The single-core performance is very low, and there are usually at least a few linking stages in any source code compilation that collapse the process down to a single thread - you can't beat Amdahl's Law here.
See? Always surprising.
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.
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 ;-)