Thank you everyone that contributed to reporting this issue. We were able to implement a possible fix that will improve performance drastically when many associations with slower data types are involved. This improvement will be available in version 6.3.0.

Some performance difference might still be present comparing directly with Hibernate 5, though, as the way we construct queries, join associations and in general fetch entity data has changed quite a bit with version 6. One general recommendation to get around this would be avoiding mapping associations, especially when to-many, as FetchType.EAGER and instead explicitly specify what to fetch for each query or load.