Assume in your computer, the most of your daytime processes spend 75% of their time running in the CPU and 25% waiting for service from disk. Suppose also that someone is trying to sell you a processor array upgrade that is 50% faster than what you have and cost $10,000. The day before, someone called had called you on then phone offering you a set of disk drives for $7,000. These new disks promise two and a half times the throughput of your existing disks.
You know the system performance is starting to degrade, so you need to do something.
Which would you choose to yield the best performance improvement for the least amount of money (i.e. based on the dollars spent per performance improvement percentage point)? Why?