The Pentium, for example, is a CISC processor. A CISC processor has a large, complex set of instructions that the processor can execute, whereas a RISC processor has a small set of simple, fast instructions. The idea behind RISC is that a if you reduce the number of instructions a processor understands, the remaining instructions can be run much more efficiently. Since most processors are performing a few tasks all the time and other tasks quite rarely, you can get a big performance boost by removing those rarely-used instructions. While it'll now take several steps for the processor to perform those tasks, they're used so rarely that the benefit of streamlining the most common instructions outweighs the loss of speed in the complicated instructions.
Processors can execute millions of instructions per second.
However, that 400MHz processor isn't necessarily completing 400 million instructions a second. Additionally, the processor sometimes takes more than one cycle to complete an instruction. Too many complex instructions defeats the general goals of RISC architecture. It's important to note that neither RISC nor CISC is an inherently better scheme. They both have their advantages and disadvantages.