The disk locality speed-up is increasingly not due to bandwidth, but latency (directly related to physical distance), which still matters.<p>To provide some perspective - in 1 cycle of a 1GHz CPU  light (or electric potential) travels about 30cm. So round-trip communication with a disk (or whatever) that is 5cm away vs one that is one 1m away (could be same rack) would take 20 times longer in each direction. Now consider communicating across a football-field-sized datacenter.