Actually realtime doesn't mean "responds faster than the fastest it needs to" but rather it means always responds faster than some predetermined threshold. A lot of the easier ways to make things fast (like caches, or even branch prediction) are inherently probabilistic; if the thing is in the cache you get wonderfully fast performance; if not performance falls off a cliff. A realtime system avoids that, making the average case much slower but the slowest case much faster. It basically flattens the latency distribution.