From Jan 2016:

https://queue.acm.org/detail.cfm?id=2874238

For the entire careers of most practicing computer scientists, a fundamental observation has consistently held true: CPUs are significantly more performant and more expensive than I/O devices. The fact that CPUs can process data at extremely high rates, while simultaneously servicing multiple I/O devices, has had a sweeping impact on the design of both hardware and software for systems of all sizes, for pretty much as long as we've been building them.

This assumption, however, is in the process of being completely invalidated.
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Even today, a Spark committer says that for Spark CPU is already often the bottleneck (for decompression):

https://www.oreilly.com/ideas/investigating-sparks-performance

On the other hand if you do more IO, you can have higher CPU load due to compression and serialization.