Disk IO is not always fast. Memory bandwidth is a huge bottleneck on current-day systems, especially on high-frequency, high-core-count processors. And so on a

Disk IO is not always fast. Memory bandwidth is a huge bottleneck on current-day systems, especially on high-frequency, high-core-count processors. And so on and so forth. If you want a snappy and truly usable system you still need to care about performance issues, particularly given the rise in non-reducible system requirements e.g. for browsing the modern web.