If the cache is slower than your CPU, your computer will be slow. But if it is faster, your computer won't speed up. So you want to make sure the cache is fast enough for your computer, but getting faster cache memory is a waste of money. Second, the amount of cache memory also affects the speed of your computer. In general, the more cache, the faster your computer will go.
Most computers have a fairly small limit on the amount of cache RAM possible.
Generally, make sure your computer has as much cache RAM as it can handle. A cache is a short-term storage place. Main computer memory is way slower than microprocessor chips, and they can spend most of their time waiting to get a value from memory to operate on. You could speed up all memory, but this costs lots of money and power. If you can only afford to speed up a small fraction of the total memory, it's a good idea to arrange for the computer to use the fast memory more often than it uses the slow memory.