Mild:
* Flop: A general term for something that fails to succeed, including performances.
* Stinker: A performance that is particularly bad.
* Dog: Similar to "stinker", but implies a lack of quality and interest.
* Bomb: A performance that is met with negative reactions from the audience.
* Train wreck: A performance that is chaotic and poorly executed.
* Cringer: A performance that is so bad it makes you cringe.
More Harsh:
* Disaster: A total failure, implying complete lack of success.
* Turkey: A performance that is incredibly bad, often used humorously.
* Piece of crap: A very strong insult, suggesting the production is worthless.
* Garbage: Similar to "piece of crap", but more commonly used in informal contexts.
* Car crash: A performance that is so bad it is almost entertaining to watch because of its disastrousness.
The best slang term to use will depend on the context and the level of formality you want to use.
Remember, these terms are often subjective and can be considered offensive depending on the situation. Choose your words carefully!