This article is a simple illustrative introduction to Amdahl's Law.

"Indeed, just as with parallel processors, there is a point of diminishing return. Adding the first 10 riding mowers reduced the time by 36 minutes. Adding another 30 only saved me 4 minutes. Adding 100 mowers makes little sense since I’ll never get below 20 minutes. (Although I would love to see such a lawn mowing demolition derby — in my neighbors yard of course.)"