A CPU running at 3 GHz executes an instruction in well under a nanosecond and waits around 100 nanoseconds for main memory. What does that ratio mean in practice?

B. That memory access patterns matter more than operation counts

A hundred nanoseconds is roughly 300 instructions the processor could have executed instead, so a miss dwarfs the arithmetic. That is why an array often beats a linked list with the same big-O, and why memory layout is a first-order performance concern rather than a detail.