Expand on advice for how to find resource usage problems

[jkrick]

There is already something here under "Optimizing code for **"", but it would be good to point users to more tips on how to find places in their code which disproportionately use resources.

eg., " my code runs slower on Fornax then on my laptop" - we could advise that user how to

1. find the bottlenecks in the code (examine output of top), and
2. remove those bottlenecks (parallelization)

eg., "my code is slow, how do I speed it up"

• same answer as above