Cache and performance

Cache is a small amount of very fast memory inside the CPU. It keeps data and instructions that are used often, so the CPU can get them quickly instead of fetching them from RAM.

RAM is much slower than the CPU. If the CPU needs something that is not in the cache, it has to fetch it from RAM, and while it waits the CPU can sit idle. This waiting is what slows processing down.

Why a larger cache helps

A larger cache size can hold more data and instructions. That makes it more likely that what the CPU needs next is already in the cache, so fewer slow fetches from RAM are needed. Less time spent waiting means better performance. A smaller cache holds less, so more requests miss and go to RAM, adding waiting time.

For example, if a program repeats the same short set of instructions in a loop, those instructions can stay in the cache while the loop runs. The CPU does not need to keep collecting them from RAM each time.

Limits of cache size

Cache is costly to include in a processor, so only a limited amount is provided. Adding more only helps if it stores the data and instructions the CPU actually needs; past a certain size, extra cache makes little difference. Also, cache does not make the CPU itself faster or give you more storage for programs. It reduces waiting time by making trips to RAM less frequent.