What to DRAM latencies mean?

While CPUs sped up tremendously, at least in MHz and throughput terms, over the past three decades, we can't say the same for the memory. In the mid-80's 80286 PC-AT days, the high-end PC memory ran at the same speed as the CPU, up to 12 MHz. Then, six years ago, when the first 1+ GHz PCs came out, the fastest memory on them was 133 MHz SDRAM, with 1/8 the clock of the CPU.

Now, things look a little better bandwidth-wise, with DDR3 routinely providing throughput at around half the CPU frequency - a 3 GHz Core 2 Extreme may be fed by DDR3-1500 memory, for instance.

However, despite the bandwidth improvements, it is memory latency, the time it takes to write or read the first word in a transfer in particular, that had advanced at a snail's pace all this time. And, it can impact the performance, despite all the caching, streaming and other "optimisations".