I’m not saying automapper is the problem, im saying we have other tools. The perfomance issue depend on you business, application, and your customer.
Having automapper, will speed up your development. But be in mind you have trades off.

The problem with automapper most of the time is not how automapper works, is how people use it, like having a bunch of business logic in the automapper is wrong, lead you to run time problem, debugging that type of problem is madness. Thanks!