I don't see why performance would be significantly different between multiple threads using same sqlite db vs multiple processes on same machine. Can you explain more what you mean?
Maybe i misunderstand, but it doesn't seem like the linked document supports what you are saying. The linked docment also doesn't describe how things work in WAL mode which is how most people would (or should) use sqlite in production.
https://www.sqlite.org/lockingv3.html
In the single process access model, you can connect exactly once and remain in a reserved lock state the entire time.