Page "Amdahl's law" Paragraph 21

Assume that a task has two independent parts, A and B.

B takes roughly 25 % of the time of the whole computation.

By working very hard, one may be able to make this part 5 times faster, but this only reduces the time for the whole computation by a little.

In contrast, one may need to perform less work to make part A be twice as fast.

This will make the computation much faster than by optimizing part B, even though B's speed-up is greater by ratio, ( 5 × versus 2 ×).