It's probably better to look at things proportionally, rather than in terms of absolute time saved. And it's definitely more useful to consider terms like "IO b

It's probably better to look at things proportionally, rather than in terms of absolute time saved. And it's definitely more useful to consider terms like "IO bound" and "CPU bound" as relative and approximate, rather than unattainable absolutes. Using the original numbers of 5 seconds IO, 1 second CPU: If doubling the speed of the CPU portion only makes the overall process 8% faster, it's not meaningfully CPU bound. But doubling the IO speed would reduce the total job time by 41%, which is a pretty big speedup overall. Using your example of 5 seconds IO and 0.05 seconds CPU: doubling the CPU speed gives a 0.5% overall reduction in job time, and doubling the IO performance gives a 49.5% overall reduction. So this example can be meaningfully said to be more IO bound than the first example.