At the very least, there will be some performance degradation. How much depends on implementation. Implement poorly and bad things happen.
However, balancing the loss of performance (however minor) is the increase in maintainability of your software.
You aren't spending countless hours developing and testing a home-grown solution.
The folks who developed Hibernate have already addressed (and mitigated) many if not almost all problems any home-grown solution seeks to address.
Plus, Hibernate is an accepted standard, well understood by the industry. You can hire engineers with expertise in Hibernate. With a home-grown solution, you can't.