Does the -j option really matter?

In class we decided to make a script t0 build a certain big package multiple times using different -j values to see if they had any affect on the time it took to build the package. I chose the pingus package. Despite the idea that using more cores to do the job should theoretically speed up the process, my output shows quite the opposite. Running the same -j value three times came up with different results each time. The fact that these results were different each time could probably be due to the fact that other people may have also been using the resources of my specific computer at the same time. As you can see in the bar graph the average time taken to compile this package is around 5 minutes and this doesn't seem to change no matter what -j value is used. The remaining two build runs for -j9 were not done because I lost connection, however due to the trend it is safe to assume they would have been the same.

For the pingus package, the -j value does not have a significant effect.