I recently used JProfiler on a slow website and it turned out that Hibernate classes were hogging a most of the resources/memory.
There are whole books on dealing with just Hibernate issues.
Does your book address the issues of 3rd party libraries in general on performance and how to minimize the impact?
In all seriousness, Hibernate is somewhat notorious as a resource hog.
But be careful when using profilers - they should only be used once you've determined that the application code (& libraries) are the source of slowness.
Now, in your case, they might be but a profiler would probably still point to them as being the problem even if what was really causing the issue was GC, or the database being slow.
The book gives some good heuristics for when it's a good idea to use profilers (& when it's not) - Spoiler: Profilers are much less useful than most developers believe, and they can seriously mislead you if you're not careful & take the time to understand how they work & their shortcomings.