In addition to performance, I would switch when the idea is to have the whole codebase, or a specific part of it, typechecked at "compile time". This way it can be part of a ci pipeline or a precommit hook, instead of blowing up with a good descriptive error (typeguard) but at runtime.

Typeguard has a performance hit as it runs at runtime, on the other hand it provides better type safety than mypy.