... and may also break compatibility with previous 10.Y releases if required for later cleanup work.
If you read through the whole paragraph, it is clear that they mean the compatibility of previous jellyfin versions.
Also, again:
Note however that the 10.Y.Z release chain represents the "cleanup" of the codebase, so it should be accepted that 10.Y.Z breaks all compatibility,
That means that the code is not cleaned up with that release.
If you would release 11 before the code is considered cleaned up, you would basically break your own defined versioning convention. That is best decided by the active maintainers.
Performance is not the goal, but cleaner code and more manageable code. But both will ultimately lead to better performance. As of now it was basically impossible to change something in the database structure since it was hard to estimate the impact of it.