This one will go on and on, but the difference is actually quite simple (though not in practice):
poor performance, and its more important cousin, incompetence, can not be ‘warned off’. A person who is incompetent to do the job can not be warned or threatened to do a proper job. Simply put, you hired the wrong person. You might be able to fix that with training, but more often than not, it cant because the mistake was made much much earlier, at the hiring stage.
Misconduct, however, is behaviour the employee can control and change, hence a warning is a proper tool for the employer to stop it. Misconduct is also, by its very nature, always deliberate. No one accidentally commits fraud. Or comes in late (on a regular basis) or forgets to file reports.
You can qualify minor instances of misconduct as poor performance (as Minters are doing in their overview here http://www.minterellison.co.nz/Problems_with_poor_performance_-_How_to_performance_manage_your_employees_and_get_it_right_03-22-2016/ ) .But it misses the point because the management response to poor performance is a performance management plan. How do you performance manage someone who comes in late all the time? Or files (perfectly fine) reports always after the deadline? When you think about it, those are then not performance issues. Its misconduct and the proper response is a warning.
Of course, one has to always investigate and look closely at what is happening. The employee who always comes in late might have problems at home. A newborn can upset the daily routine greatly and for a while means that person is just not operating at their best. But every good manager knows about those life changing events among their staff anyway.
So, I don’t think I am going too much out on a limb here by saying that the majority of poor performance cases are actually misconduct scenarios. You only ever have true poor performance or incompetence if you hired the wrong person or the right person for the wrong job.