Actually we have been spending a lot time with Amdahl's law already. Software people assume it only applies to parallerizing software. In "computer organization & design" By Patterson and Hennessy it is stated in the general form. And example is used how much multiply unit should be sped up to get five fold improvement in execution time when 80% of time is spend in multiplication?

Execution time after improved=

(execution time affected by improvement / amount of improved)+Execution time unaffected.