Amdahl's Law
Execution Time After Improvement = Execution Time Unaffected +( Execution Time Affected / Amount of Improvement )
Example: "Suppose a program runs in 100 seconds on a machine, with multiply responsible for 80 seconds of this time. How much do we have to improve the speed of multiplication if we want the program to run 4 times faster?" How about making it 5 times faster?
Principle: Make the common case fast