Details here: https://github.com/LemmyNet/lemmy/issues/3165
This will VASTLY decrease the server load of I/O for PostgreSQL, as this mistaken code is doing writes of ~1700 rows (each known Lemmy instance in the database) on every single comment & post creation. This creates record-locking issues given it is writes, which are harsh on the system. Once this is fixed, some site operators will be able to downgrade their hardware! ;)
This is such a dumb take. Using a database efficiently is not some binary, once-off thing: you build what works based on the data you have at the time. When it works, you move on to other features. It takes analysis of real operation over time to find the bottlenecks, and discipline to focus on fixing the things that will have the most benefit to your users.
There are many successful tech companies who introduce features that create dogshit performance impacts regularly. They work because there are people looking at metrics and catching issues to fix. This is healthy.