It's not about memory/CPU/IO, but latency vs throughput. Most software is slow because it ignores the latency. If you program serially waiting for _whatever_ it is going to be slow. If you scatter your data around memory, or read from disk in small chunks, or make tons of tiny queries to the DB serially your software will be 99.9% waiting idle for something to finish. That's it. If you can organize your data linearly in memory and/or work on batches of it at the time and/or parallelize stuff and/or batch your IO, it is going to be fast.