An upgrade from mongo-java-driver (and mongo-java-driver) from 4.1.1 to 4.2.0 lead to increased memory consumption and decreased performance. (Jobs taking massively longer than before)
We solved the issues by rolling back to 4.1.1.
We don't have minimal reproducing examples but it is very visible for usecases where we stream a huge amount of documents reading from the database using reactive streams.
The changelog states that you exchanged the internal reactive-streams implementation so maybe it has something to do with this.