Page "Amdahl's law" Paragraph 2

The speedup of a program using multiple processors in parallel computing is limited by the time needed for the sequential fraction of the program.

For example, if a program needs 20 hours using a single processor core, and a particular portion of 1 hour cannot be parallelized, while the remaining promising portion of 19 hours ( 95 %) can be parallelized, then regardless of how many processors we devote to a parallelized execution of this program, the minimum execution time cannot be less than that critical 1 hour.

Hence the speedup is limited up to 20 ×, as the diagram illustrates.