Do you know what the motivation for this over just-using-postgres was? I’m confident you can beat postgres if you’re careful and thoughtful, but I’m also reasonably confident rolling your own system like this is at minimum going to be brittle, and probably have at least as many performance pitfalls at the end of the day.
Discussion
I wouldn't use Postgres again after how much of a pain it was in Pleroma. The better question is why not just use LMDB if you're going to go through all this trouble. They're equally hard, but at this rate LMDB would be easier.
I've read that sqlite is significantly faster for read-heavy applications.