From section 4 of Chapter 3 of this resource, study the section titled "Amdahl's Law."
1 speedup =  1  P
1 speedup =  P + S  N

speedup  N P = .50 P = .90 P = .95 P = .99      10 1.82 5.26 6.89 9.17 100 1.98 9.17 16.80 50.25 1,000 1.99 9.91 19.62 90.99 10,000 1.99 9.91 19.96 99.02 100,000 1.99 9.99 19.99 99.90
"Famous" quote: You can spend a lifetime getting 95% of your code to be parallel, and never achieve better than 20x speedup no matter how many processors you throw at it!
2D Grid Calculations 85 seconds 85% Serial fraction 15 seconds 15%
We can increase the problem size by doubling the grid dimensions and halving the time step. This results in four times the number of grid points and twice the number of time steps. The timings then look like:
2D Grid Calculations 680 seconds 97.84% Serial fraction 15 seconds 2.16%
Source: Blaise Barney, https://computing.llnl.gov/tutorials/parallel_comp/
This work is in the Public Domain.