Current processors run at very high MHz internally, while externally they are much slower. e.g. on a 200 MHz processor, only what can fit in the internal (or L1) cache of 32/64 kbytes runs at 200 MHz; bigger programs/data have to be read from the secondary (L2) cache of 512 kbytes, which operates at only 66 MHz (much higher on Pentium II); real big programs/data which cannot fit in the L2 cache have to be read from main memory of 16/32/64 Mbytes. (Of course Windows 95/NT programs won't even fit in the main memory and will swap to the hard disk!).
That's why main memory and L2 cache read/write/transfer rates (in short bandwidth) are so important; and a 233 MHz processor is less than 5% faster than the same one at 200 MHz!