Instruction cache's purpose
In the words of Father Ted, the instruction cache is small but the memory is far away.

The 25Mhz memory will take at least four of the 100MHz processor's cycles. The instruction cache does not actually make the access to main memory faster, but it will return values immediately rather than after a delay. The advantage of this is on the second and subsequent time the processor attempts to access a particular address. Such as when executing a loop.