A CISC (Complex Instruction Set Computer) processor has many instructions, some of them complex instructions that do a lot per machine cycle. The Intel processors are CISC. A RISC (Reduced Instruction Set Computer) processor has few instructions, most of them simple. It may take several instructions to do the equivalent of a CISC instruction. The MIPS processors are RISC.
If everything else were the same, CISC would have greater throughput. However, a larger silicon chip is needed to implement the many complex instructions. This means that data must move through greater distances, and that takes more time. So the machine cycle must be slower. Also, the instructions themselves take more time to execute (usually more than several RISC instructions). Fine tuning the chip for speed is difficult when there are many instruction types. The simple instructions are compromised by the complex ones. But the simple instructions are the most frequently executed!
Worse, it is hard for a compiler to make use of complex instructions. They frequently compile programs into machine programs that use only simple instructions. The MMX (multi-media extension) instructions added to Pentium chips (in 1997) are not output by any compiler. (In 2015 compilers still have trouble with these instructions.)
If you are writing a computer game and wish to use MMX instructions, what must you do?