Object-relational mapping (ORM) libraries are popular due to their usability benefits but come with important performance tradeoffs that are often poorly understood and quantified. To evaluate the performance of major ORM libraries, a new benchmarking tool called IMDBench has been open-sourced. The results show that ORMs have some unfortunate characteristics such as requiring multiple queries to fetch all the data needed for complex operations. This is why they are perceptually slower than raw SQL. However, it's important to understand the limitations of ORMs if you wish to make an informed decision regarding these technologies.