[docs] add section on debugging performance issues

[odow]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (5975249) to head (9950916).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3850   +/-   ##
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  Coverage   97.54%   97.54%           
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  Files          44       44           
  Lines        6017     6027   +10     
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+ Hits         5869     5879   +10     
  Misses        148      148           

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[blegat]

I also find it helpful to just call attach_optimizer but not optimize! to see which part is copying the model

[odow]

I chose not to mention attach_optimizer. It's a bit too complicated for a common user question in a getting started tutorial.