When people talk about Doctrine (or any ORM for that matter), the performance issue always comes up fairly quickly. Besides the fact that Doctrine will help you develop faster, so a little overhead doesn't really matter, there are numerous options to increase the performance of the application.
By understanding how the system works in the first place, a lot of issues can be avoided right away.
When you have done everything to avoid these pitfalls, you can bring in the big guns: caching. Doctrine has several caching mechanism and since Doctrine 2.5 "Second Level Cache" was added to our toolbox. After this talk, you should know what the impact is of every cache and how to use them.