How much computing power is needed to keep commercial and military planes flying?
For commercial airliners: the computing power is set during development and certification of the type. The A320 was developed in the 1980s and still uses Intel 80186 and Motorola 68020 processors. Your i7 CPU would be able to run rings around anything on board of the average airliner.

But processing capacity is of secondary importance to airliners, safety and redundancy are in the limelight. Any processor capacity that can do the job is good enough, and is only the start of all the aspects that go into certifying an airliner including its systems. 

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Military planes are different, in that the requirements are more pressing and the accepted risks are higher. Yes modern fly-by-wire planes use digital systems, however when the F-16 first came out with a fly-by-wire control system, it was a quadruple redundant analog system, with a digital processing requirement of zero. These were upgraded to digital fly-by-wire in the block 40/42 upgrade in the early 90s, so again the fly-by-wire used pretty primitive processors by today's standards.

Generalising, the long development and certification/ready-for-mission time of aircraft systems means that any system on board uses processing power that is behind the latest-to-market specs.