Gene Amdahl's 1967 brilliant and elegant insight fundamentally changed how computer scientists think about system optimization: the performance improvement of any system is ultimately limited by the portion that cannot be enhanced. If only 30% of your system can be accelerated, even perfect optimization of that portion will never improve overall performance by more than 43%.
A great example of Amdahl's Law in action in a very frustrating place that so many can relate to: https://viewfromthewing.com/83-billion-wasted-showing-up-at-the-airport-3-hours-before-your-flight-is-a-system-failure-no-ones-trying-to-fix/