It's not just about performance: having an in-process MVCC engine would simplify the implementation of many single-machine concurrent applications. Currently you usually have to combine SQLite with some kind of concurrency primitives; this is extremely painful because most OS-level concurrency primitives are really easy to misuse (e.g. it's trivial to accidentally add deadlocks, and very hard to spot and remove these ahead of time: example hard to spot concurrency bugs https://fly.io/blog/corrosion/, https://rfd.shared.oxide.computer/rfd/400)
