In both cases the difference was energy use. Airplanes could be a lot faster and more powerful before the oil embargo, when everything shifted to save more fue

In both cases the difference was energy use. Airplanes could be a lot faster and more powerful before the oil embargo, when everything shifted to save more fuel. With CPUs the story is similar, we could have kept going on to 6/7 GHz but the energy constraints, both of supplying that energy and dissipating the resulting heat, would have been prohibitive. Even now, things are still changing, we have better liquid cooling and even more commercially viable phase change cooling. These allow 5 or 8 GHz, respectively. But most of all, parallel computing is in much better shape, both theoretically and practically, it's a much more viable approach now than it was 15 years ago.